Leveling a bathroom floor - toilet in the low spot
Depending on how large of an area is sunken, you could cut up the old plywood with a circular saw. Sister boards to the joists and add cross members to the cavities to raise the area to match the surrounding floor. Follow up with plywood and then cement board before tiling and you should have a solid, fairly level floor to work with. This can all be done  easily even though the basement is already finished. This seems preferential to any floor leveler, since you'd actually be addressing the problem, which is a sunken floor.

It would also be a good time to inspect the existing floor joists and see how damaged they may be. If the damage isn't extensive, doubling them up my suffice, or you may need to replace larger sections entirely.