I always thought it stood for "Set Sail", but this definition is more pragmatic.
(Officially USSS, but it's often abbreviated to SS in common parlance.)
Apparently the RMS prefix on the RMS Titanic stands for "royal mail steamer" as it was authorised to carry the post.
It's not unusual for acronyms to have multiple meanings, the S in HMS can mean submarine, as submarines aren't ships.