I cruised around on a sailboat for a decade, so I would just like to say “the sea is a harsh mistress”. A floating hulk like an old tanker or freighter would be a good starting point but anything smaller would not be very comfortable, IMHO. With a big hulk you would have some acreage for solar and some mass to reduce wave motion and some living space. Of course you might even inherit some infrastructure plumbing and wiring but those are higher value items that could have been stripped. You would have to invest in some pretty reliable pumps and spares to keep the sea out – she will find every weakness. Scantlings on older ships are usually more in your favor – a harbor pilot once told me they don’t use any more steel to make a Panamax container ship than they did to build a WWII liberty ship!