Marine ply is usually the sub base to lay the actual wet room flooring onto.
The area that you actually stand on to bath is a fixed composite floor panel with a built in waste and has a natural run off to the waste.
There are many ways to lay a wet room floor, but the most common way is to set a composite floor panel into your existing floor .
Basically you cut away the size of the panel out of your existing floor, then sit the panel base into the cut out , so its now sitting flush with the existing floor.
The new rubber flooring is then laid on top of this , and turn up at the edges to form a water tight area.
It all depends on what type of floor finish you want ?
The above installation would be used for a rubber/vinyl type flooring, a tiled floor would be laid differently, perhaps forming your own run into a waste, or using a specific base manufactured to go under a tiled finish, but again you would still use marine ply to lay the tiles onto for the rest of the room.
Its not just a case of dropping in your composite sub base into the floor cut out, and then tiling the whole area, one rule is you NEVER EVER tile on top of floor boards, you always screw down marine ply onto existing floor boards.
Heres a good example of how you would prepare the floor for a tiled wet room finish.
https://www.youtube.com/watch?v=IIQyiESIWQE
Basically each type of finished floor would have a different way of preparing the under base.