What would've happened if they'd then sat there as a group, used your generation method, & made practically the same characters?
I would have been pretty darn surprised.
Possible, but, extremely unlikely since the chargen system I was using would directly tie them to the setting and each other by working as a group to create backstories where every character would be tied to another character in some way and tied to the setting. IOW, they might have had the same classes, but, I really, really doubt they could have created the same characters.
Nobody needs to be a sailor in GoSM. They just need a little money or to go on long walks.
Also - someone can spend their entire life working along the sea (fisher) and not have the slightest clue how to navigate a larger vessel once they are out of sight of land. Or they could be miners, or bakers or.. whatever.
Umm, well, maybe you run it that way, I don't. The first adventure sees the party get a sailing ship. Most of the adventures require you to sail somewhere. In fact, the only adventure after the first one that you could possibly do without a ship is Danger at Dunwater and even then, you're expected to sail to the location. The other eight adventures all require a ship. Unless the PC's are always just passengers, I suppose.
But, yeah, if your first reaction to someone pitching Ghosts of Saltmarsh, the adventure book with THIS as the cover:
is, "Oh, no one needs to be a sailor or have any nautical experience", then again, I'm willing to go out on a limb here and say that I'm not the issue here. I am very sure that you would not enjoy playing at my table.