Is it bad that when I finish in the loo, and open the door, my cat instantly comes in to drink toilet water? Then my dog does after the cat is finished. Is toilet drinking normal, especially if they drink like half the water in it?
No, you should give them more water normally and punish them (water sprayer or something) when they drink from the toilet, it can end up with very bad infections, or, if you clean it properly, bleach poisoning.
Make sure their water bowls are full of cool water (try putting some ice in it) because, like we do, they prefer it when it is cold.