Unfortunately there is no way to "floodproof" wood building construction that I know of. Unless you can seal off every possible entry point of water from the exterior, "water will always find a way".

First of all, on top of the plywood, you only need cement board OR Ditra. They are both underlayments. If you use Ditra (I much prefer it, as it's way easier and faster to lay), and tape the seams, it will waterproof the floor.

***BUT***

That's waterproofing from someone spilling on the floor, not from a flood that rises from below or an overwhelming flow of water that sweeps in from the outside. Waterproofing in bathrooms is done to keep water from getting into the walls from the inside the room. A flood does the opposite. I'm afraid floorman's right: you can't flood proof a building unless you build a dike around it.

You need to have licensed and insured contractors doing any work on your building. No homeowner DIY allowed on multi-family dwellings. You don't have the insurance to cover yourself if anything you do is wrong, and neither does your building.

You can't control flooding. If it's gonna happen...it's just gonna happen. You might consider having one or two linear drains installed, with an adequate slope to one or both, at least the water wouldn't just sit there for days.