In Once Upon a Time, season 5, episode 11, "Hook says, 'Look. The S.S. Purgatory', as Charon emerges from the pond's heavy mist aboard his boat, ready to take the prisoners down to the Underworld." what is the meaning of "The S.S. Purgatory" in this context? Based on my search, it seems S.S. stands for Straight Spouse, but I am not sure if that's correct. Thank you!
S.S. is short for "Steam Ship" (or perhaps "Screw Steamer"), and is a common part of the name of many ships, typically large luxurious passenger cruise ships. (For example the S.S. Titanic). So Hook is referring to a large steamship named "Purgatory".
Purgatory is, in Christian belief, a place where souls go after death. It is a place of punishment for those souls that are sinful, but not so sinful that they must go to Hell, and after some time in purgatory, those souls can enter heaven.
Looking at the source, this is ironic, as the boat is not a large cruise liner, but the punt or rowboat of Charon.