You can try the Schluter water proofing method under the tiles, but if you
have that much water maybe you should slope the floor down to a drain and
add a "dam" across the door threshold to keep the water in.

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In a typical bathroom floor tile installation you have the following
sequence:

Plywood subfloor, Thin-Set, Backerboard, Thin-Set, Tile/Grout.

If the grout is not perfectly sealed and the bathroom is heavily used
(with a lot of water on the floor), where does the water stop? I know
Backerboard is water-damage-resistant, but I hear it can wick and
absorb water. What are we relying on to keep the water from soaking
the plywood? Is it just the thin-set?