What can I do about a low spot in my bathroom floor?
Why do you expect your floor to drain or dry? Is this a wet room, where the whole floor area is designed to get wet and drain? In most bathrooms (at least in the US), only the bath/shower itself is designed to drain water. The rest of the floor is typically made of a material that won't be damaged by an occasional splash but also isn't designed to drain large amounts of water.

If you have a wet room with areas that don't drain, then the solution is to fix the shape of the floor so those areas do drain. The exact approach (and how invasive the repair would be) depends on your particular layout and materials.

If you have a more typical bathroom, then you should not expect the floor to function as a drain, and instead work to prevent it from getting quite so wet. Often this can be solved with better placement of shower curtains and bath mats or repair of leaks. For advice on that we'd need to know where this water is coming from.