I don't know if GRP sheets would be better, it's what they build somd lorry bodies and things like burger trailers out of. It's marine ply but with some sort of formica/plastic stuff bonded to the outside. Very strong quite expensive.
Maybe you could insulate, build with ply wood, ands then use sheets of fibreglass and brushable resin to seal the space. I just had the roof of a very leaky horse box repaired that way. It's 'rowing boat' technology, so it's gonna be water/damp proof. Messy and nasty to do though.
What about Sealing all the joints with quality silicon in a tube, then painting the walls and floor with swimming pool paint? It's thick, would fill up all the seams that the silicon didn't get, and I'm sure it comes in white. If it stops water in a pool getting to the concrete structure, a little humidity in a room even spilt resevoirs shouldn't be a problem.