While the ... --- ... is technically the morse code equivalent of "SOS", it's not actually the universal signal for distress. The real SOS is actually a special signal code, which, expressed in print looks something like:
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SOS
What the bar over the letters means is that instead of sending the letters seperately, like:
dididit dahdahdah dididit
The entire sequence is sent as one unit, like:
didididahdahdahdididit
This system of signals is an integral part of International Morse, and gives these codes a specific meaning. For example, sending the code:
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SK
means to another operator that it's the end of your last transmission to him. The letter K alone means "back to you", and the code SK with a bar means "back to you and I'm done."
But, like the SOS, the SK isn't sent as the letter S and then the letter K, but as a solid unit. For example, instead of:
dididit dahdidah
the radio operator sends:
didididahdidah
Will sending a dididit dahdahdah dididit let people know you're in distress? Absolutely. Is it technically correct? No.
In case you're wondering, I use the written out dit dah form of morse code instead of the dots and dashes because it's less of a mental crutch to people trying to learn the code later in the process. Morse code is a totally aural system these days, and learning a print version instead of associating the sounds directly with the letters will only slow you down later in the learning process. I know because I thought I'd be smart and teach myself, and now I can't listen to anything over 10 words per minute.