Why do I get really thirsty when I have a bath?

It could be that you're having a very hot bath and when you have a very hot bath your body temperature goes up very high...

Normally you would regulate body temperature by sweating. When you sweat water moves onto the surface of your skin. That water evaporating then carries heat away from the body's surface. That's called latent heat of evaporation and when water evaporates in this way it carries away extra heat from you and cools you down.

If you're in a steamy bathroom it's much harder for the water to evaporate, and, also, if you're submersed in the bath it's impossible for the water to evaporate from that surface so your temperature goes up. This means you can actually end up feeling as though you've lost lots of water because you're so warm and the body's reflex is to encourage you to drink.