Seagoing ships are good at being in salt water but don't deal as well with being engulfed in plasma. Starship is pretty good at being engulfed in plasma but doesn't handle being engulfed in salt water very well. Making a single ship that can handle both is a very tall order, even just doing one or the other is hard.
Seagoing ships are pretty good at floating on top of salt water for a while, but they're not very good at being "dunked" in it (i.e. sunken). It is possible to raise sunken ships and rehabilitate them to regular service, and this has been before, but it generally destroys everything inside, and requires a ton of work. Salt water is one of the most corrosive substances we know of.
