I used this glass fiber composite material, Thermo-Lite, for the floor in my van conversion. Strong, light, doesn’t absorb water but it is expensive.
"Our material is a unique closed cell, lightweight composite product, manufactured with cross-linked polymer foam and fiberglass, that offers high specific
strength and toughness. The non-absorbent material is extremely durable,
provides and excellent bonding surface, has good impact strength, sound and
thermal insulation, and is resistant to contamination. Offered in sheets or cut
into parts, our material is built for lifetime performance, and is ideal for endless coring applications subjected to static and dynamic loads."
https://www.spaceagesynthetics.com/