Rapaire, my pal's son just sailed to Fiji in his ferro-cement yacht. Quite a common building material.
It has been raining there for three weeks, he says.
Some landing beaches for WW2 D-Day had no harbours, so they built concrete ones and towed them over and sank them into place.

In WW2 the British experimented with ships built of Pykrete, which is sawdust frozen in water. It worked, but the war ended before it was used, to the relief of brass monkeys. The formula is in Wikipedia, if you want to try.