It turns out that if you don't need your ship to go fast, all you need to do is have a structure that can produce enough displacement to be bouyant and stable. You could carve a ship out of marble if you wanted.
https://en.wikipedia.org/wiki/Project_Habakkuk
https://en.wikipedia.org/wiki/Great_Pacific_Garbage_Patch
https://mythresults.com/alaska-special-2
It turns out that if you don't need your ship to go fast, all you need to do is have a structure that can produce enough displacement to be bouyant and stable. You could carve a ship out of marble if you wanted.