The Marconi companies adopted C.Q.D. as a distress signal in 1904, and the next year Germany chose SOS as its distress signal. A year later SOS was adopted by the 1906 Berlin Radiotelegraphic Conference as the international standard, but it took a few years for many Marconi operators to grudgingly switch to the new distress call.
The Yearbook of Wireless Telegraphy and Telephony, 1913, pages 318-322:

AS navigation has developed from the earliest times, means of signalling from ships to the coast and to passing vessels have been devised and improved, and have been operated under an admirable organisation, but the systems upon which they worked reached their conceivably practical limits long before the invention of Wireless Telegraphy.
    Any one of the several systems depended either upon vision or upon propagation of sound, the former being the earliest known.
    Lights, flags, rockets, guns, and sirens have all rendered, and are still rendering, inestimable service to navigation, but the disadvantages of visual signalling in the case of fog, and the limitations in range of visual and auditive signalling, even under the most favourable conditions, considerably restrict the usefulness of these methods.
    No one can imagine how many lives and how much property would have been saved had Wireless Telegraphy been known of in earlier days. The sight of a pirate in the good old times would not have caused so much anxiety to the skipper of the honest merchantman had the latter been able to call some other vessel to help him with the buccaneer, and no doubt some of our favourite tales of adventure, distress and rescue would never have been written. In the place of them perhaps we would have had more thrilling stories still. Here is a field for some of our novelists of to-day or of the next generation.
    It would not be correct to say that the older methods of signalling are superseded by wireless, but it is correct that wireless, with its enormous range of action as compared with that of others, and its independence of weather conditions, is now by far the first of all means of signalling, and by its own intrinsic worth alone places these other systems of signalling in the position of accessories to itself.
    When Mr. Marconi had developed his invention to such a point that its utility on board ship became obvious, the Marconi International Marine Communication Co., Ltd., was formed for the purposes implied in its title. The primary object of the new means of maritime wireless communication being to provide additional security to life and property at sea, the company have provided all its ships' stations with emergency apparatus, so that communication could still be carried on in the event of failure of any kind, particularly at the time of a serious accident which might render necessary the issue of calls for help. In this duplication of parts provision was made against the liability to interruption of the supply of electric current from the ship's dynamos, from which, in the ordinary course, power is derived to work the wireless plant, and a source of current independent of the ship's dynamos was provided as a stand-by in case of failure of the latter. Thus, almost simultaneously with the first application of Wireless Telegraphy to marine communication, the Marconi Company included in its standard wireless installations for ship purposes a suitable battery of accumulators, enabling the ship to issue distress calls, even if all the lights on board the ship were extinguished by water in the engine-room. This was over twelve years ago.
    As time went on the organisation of wireless communication at sea became more and more perfect, and it was found desirable to embody in one Circular the various directions which had been given to operators regarding the use of the apparatus in the event of accident to the ship. Thus so long ago as January 4th, 1904, the famous "C.Q.D." call was instituted by the Marconi Co. and embodied in its "General Orders." This instruction, a landmark in the history of the organisation of wireless communications, is reprinted below from the original, which is carefully preserved in the archives at Marconi House.
CIRCULAR  No.  57.

    It has been brought to our notice that the call "C.Q." (All Stations), while being satisfactory for general purposes, does not sufficiently express the urgency required in a signal of distress.
    Therefore, on and after the 1st February, 1904, the call to be given by ships in distress or in any way requiring assistance shall be "C.Q.D."
    This signal must on no account be used except by order of the Captain of the ship in distress, or other vessels or stations retransmitting the signal on account of the ship in distress.
    All stations must recognise the urgency of this call and make every effort to establish satisfactory communication with the least possible delay.
    Any mis-use of the call will result in the instant dismissal of the person improperly employing it.
                    18, Finch Lane,
                              London, E.C.,
                                        7th January, 1904.

    When the "C.Q.D." signal achieved a lasting fame, on the occasion of the wreck of the S.S. Republic, many interesting stories were spread about as to its meaning and derivation. Probably the most amusing explanation of the signal was that it indicated "Come Quick, Danger," but perusal of the above Circular will show our readers exactly how it originated.
    "C.Q." was the recognised signal used by one ship to attract the attention to it of others within hearing, so that telegraphic traffic could be commenced and transacted, and it was thought that the most appropriate distress signal would be arrived at by adding the letter "D." (denoting "Distress") to "C.Q.," the general call to attention.
    It is a great compliment to the foresight of the Marconi Company in instituting, as they did at the commencement of 1904, a special distress signal, governing its use by stringent regulations, that the International Radiotelegraphic Convention of Berlin, which entered into force in July, 1908, ratified the practice in regard to distress signals initiated in 1904.
    It is a matter of regret to some that the Berlin Convention should have superseded the old "C.Q.D." call by the new "S.O.S." This regret is shared by many of the oldest operators, and even when the new call came into force it is noteworthy that in each case of accident the "C.Q.D." call was sent out as well as the "S.O.S." The change of the call letter is, however, a sentimental regret, and "C.Q.D." is being gradually forgotten.
    It is, further, instructive to note that the International Radiotelegraphic Convention which sat in London in June, 1912, endorsed the Marconi practice in regard to emergency apparatus by deciding that all ships equipped with Wireless Telegraphy should have an emergency set as part of their wireless equipment. This prescription comes into effect in July of the present year, but as by far the greater number of the merchant vessels of the world at present equipped with Wireless Telegraphy have been so fitted little change to existing arrangements will be necessary. The United States of America, which was a party to the London Convention, gave effect to the ruling of compulsory equipment with emergency apparatus almost immediately after the London Convention of 1912 was signed.
    Much attention has been devoted to the design of apparatus suitable for distress calls, not only of special types, but also with a view to its handling by other than skilled operators. While it is true that a large number of passenger vessels are equipped with Wireless Telegraphy--and, indeed, only until a short time ago nearly all merchant vessels so equipped were passenger steamers--it is also true that a considerably larger number of cargo vessels are not fitted with this means of communication. Cargo-boat owners have hesitated to incur the cost of the equipment, and the expense of an additional person to be placed on board to operate it. Gradually these objections are being overcome by the simple question of pounds, shillings, and pence, it having been conclusively proved that this expense can be recovered many times over by considerations referred to elsewhere in this volume. With a view of simplifying equipments on small vessels (which in many cases may be able to fully justify the expense of their equipment by receiving distress calls and then proceeding to the assistance of the ship issuing them) it has been suggested that ships be so fitted and the distress call be so arranged that when issued this call should cause a bell to ring or sound a special alarm on board all ships within range.
    In the earlier days of Wireless Telegraphy, when very few stations existed, wireless signals were registered by a Morse Ink Writer, or could be made to ring a bell. This could only be done by the use of that detector of wireless signals known as the coherer. This instrument had, however, so many inherent disadvantages, the chief of which were instability, slow rate of working, and necessity for constant attention, that it had to be replaced gradually by auditive reception, and its use at the present day, even for distress purposes only, has now become impracticable. Mr. Marconi, in answer to a question put to him at the Board of Trade Inquiry into the wreck of the Titanic, has shown how a distress call could be arranged under present methods of working to ring a bell or give some other alarm in ship station at a distance, and the method he suggested is now being worked out. Instead of the "S.O.S.," which consists of a series of dots and dashes, several long dashes would be used in transmitting. The special receiver would not respond to ordinary Morse signals made up of dots and dashes, to stray signals from other vessels communicating with each other, or to atmospherical disturbances, but only to a succession of long dashes, being actuated solely by the accumulation of energy in a long sustained dash. It goes without saying that the sustained dash, or series of them, would have to be longer than any existing Morse sign, and would have to be retained solely for the purposes of distress.
    In cargo boats, where only one skilled operator is carried, the advantage of this arrangement is obvious, while in the case of any wireless station where it would be difficult to maintain constant watch at all times its utility cannot be gainsaid.
    Meantime the best possible is being done in the way of giving members of the ship's crew an elementary instruction in Wireless Telegraphy on ships where only one operator is carried, to enable them to listen at the instruments while the operator is off duty. A short practice in Morse and in the handling of the receiving instruments will enable any intelligent person with normal hearing to detect the easily-distinguishable "S.O.S.," call in the event of its being sent out. He could then at once call the operator back to the station to attend to the communication.
    We have referred above to lights, flags, rockets, guns, and sirens as means of distress signalling, and as accessories to wireless. We must not omit to mention as another and one of the most valuable accessories of the present day--namely, that of the Direction Finder, or, as it has been termed, the Wireless Compass. By means of this invention it is possible to detect, independent of weather conditions to which visual means of signalling are subordinate, the direction of one vessel in respect of another. The range of this instrument in the average mercantile equipment extends up to 50 or 60 miles. Neither must we omit to refer to one more invaluable accessory to Wireless Telegraphy in summoning assistance to a distressed vessel, and that is the submarine signalling apparatus. The apparatus is arranged with one receiver on the port side, and another on the bow of the ship, for direction finding, but as the detection of sound by this means is limited at present to between 10 and 15 miles it can only be used as an adjunct to the direction finder, and as a check upon the readings of the latter, should it be desirable to have them up to ranges within these figures.
    To describe distress signalling in all its details as it can be accomplished at the present day, and to discuss fully all its possibilities, would fill many pages more, but a general survey only has been attempted here with a view of noting the principal features. If this essay has conveyed to the mind of the reader a fair understanding of what is actually being done and what is still possible, if it has impressed upon him that science and invention are being energetically applied in this direction, under the watchful and encouraging patronage of the Authorities at home and abroad, he will be assured that everything humanly possible is being done to diminish the perils of the sea.