How do i waterproof the joists under my bathroom floor?
I've never seen or heard of this being done.

Water in a bathroon should never make it to the studs or supports in any significant supply such that it actually creates or promotes damage or rot.

The appropriate solution is to remove/stop the water source that caused the damage before it gets that far/in the first place. The flooring choices is most important in these cases. Protecting the joists wont hurt them, but is most likely wasteful from a time/cost=reward/value perspective. Spend the time and money on quality water supply, drain, and routing (removal/protective, like caulk, grout, etc) components.