JoostD wrote:... it’s prefinished with a thick layer of white paint
...
I'm just wondering if this "pre-finished" plywood might be OSB siding. If so, I wouldn't use it, but, there are some folks who have and had little problems with OSB in the short term. "Short term" is the operative phrase. Long term it just won't perform like a good exterior ply. Of course if it is some other plywood layup it may be ok. Better talk to the lumber yard and find out what it is intended for.
On another note... I've never had any issues with water intrusion through the floor. If you looked under my first TD all you see is "Black Jack" (roofing tar) spread on a 1/2" sheet of ply, no seams since I used 5x8 sheet. Other than a little dust there is no change in my seven year old TD's underside. I don't live in a wet or humid climate so take that into consideration as well.