Yeah, I was also going to say marine ply, even bunnings sells that (I saw it when looking for a piece of wood to turn my bath into a table for enlarger).
It works well enough outdoors (my dad built me a cubby house when I was a few years old, 20 years later it was still standing). But I wouldn't call it waterproof, unless you absolutely soak it in some kind of epoxy stuff. If you know how to fibreglass, I'd go that route.

Why not try one of those salvage joints, like the one on South road somewhere near Richmond road, for a cheap old steel sink? Just the weight factor?