This is something I've thought about a lot. I've always felt like there needs to be a way to swap between ships at an Outpost, under certain conditions (not in combat, no ships in close range, don't have any treasure on board, not in the middle of a Tall Tale/voyage, etc). I play SoT with a decent sized Discord group, and we run into this all the time. Friends want to join or have to leave (with no replacement), which means having to leave the server, get a new boat, and join a new server.
I wouldn't even mind if it starts you off at a disadvantage (losing some supplies, etc).
I'm not entirely sure how DOABLE this is (I don't know how they have it coded on their backend), but I'd love to see it if it IS doable.