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

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.