"SOS", the maritime distress call, has been the accepted standard distress signal for marine vessels for almost 100 years, even in these days of digital voice communications -- if it's not broke, don't fix it.
The usage of "SOS" dates back to the early 20th century, having been officially ratified in 1908. It was adopted primarily for it's ease of use and remembrance and ease of recognition: In standard Morse code, "SOS" is represented as "...---..." (dot-dot-dot, dash-dash-dash, dot-dot-dot) and is quite easy for even the most inept beginner to execute. The later words of "Save out Souls", "Save out Ship", while not intended as actual meanings, were nevertheless taught to personal as mnemonic devices for aid to remembrance and thus are technically applicable as what is colloquially called a "backronym".
Before wireless communications became common on ships, the only form of long range communications in case of lossage was signal flares or rockets which where only visible an average of 100 miles from the place of their launch. But by the middle of the first decade of the 1900's there were many long-range marine vessels equipped with Marconi wireless's and the need for a standard cross-language Morse distress code was seen.
Because being a Morse operator was a highly specialized profession, most of the maritime Morse radiomen were trained first on land-based railroad and postal telegraph systems. For landline telegraphs in Britain the signal of "CQ" was used to signal messages of crucial importance, and the signal was soon adopted for the vast majority of telegraph networks. And, quite naturally, when these trained telegraph operators were taken to sea to be radio operators the usage of "CQ" was also brought with them.
The first Meeting of the Internation Congress of Wireless Telegraphy was held in 1903 with one of their objectives to standardize a distress signal for general use on water-borne vessels. Although many proposals were discussed, there was no standard ratified. As a stop-gap measure, Marconi himself suggested the use of "CQD" (CQ from telegraph, and "D" as the standard one-letter code for "Distress"). Although is it thought to mean "Come Quickly; Danger", like SOS this is not the case but was simply an mnemonic for aid to memory.
The "CQD" signal was in general use until the second meeting of the Internation Congress of Wireless Telegraphy in 1906. At this meeting action was finally made and "SOS" as a distress signal was decided upon (but would not be ratified until 1908). The signal itself was actually an invention of the German Navy who had been using it since 1906 as their general distress call and thus had already proven it's usability.
The first recorded use my a United States vessel was the SS Arapahoe in 1909 when the ship lost its propeller. The call was heard by station "HA" and rescue was dispatched. The first use of "SOS" by a British vessel was the HMS Titanic, though it first sent out the more widely known "CQD" signal. We all know what happened that that little boat.
References:
Karl Baarslag. SOS to the Rescue
Department of Commerce, Bureau of Navigation, Radio Service. Important Events in Radiotelegraphy
Arthur Nilson. Radio Operating Questions and Answers.
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