See also: https://en.wikipedia.org/wiki/Pykrete - an alloy of the two greatest structural materials of all time, ice and sawdust!

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.

And the British attempt at a floating ice carrier:

https://en.wikipedia.org/wiki/Project_Habakkuk