Dingo said:
Had the same with mine and was advised by the vet that, depending on the food they are fed, cats can get liquid\moisture from it (ie) Mine gets loads of tuna in spring water.
There you go, just saved you £60 of vet's fees!!
Feeding lots of tuna isn't a good thing, as cats can become addicted to it and end up refusing to eat anything else. It also contains something that's really bad for them, but I can't remember what it is offhand, one of the minerals anyway.
I have a cat-it dome thing, but it is horrible to clean (it gets gunked up like an aquarium filter), and if you don't pay close attention it can run dry and damage the water.
Personally, I now just leave a tap on. My cats will only drink out of a bowl if they don't have access to a tap, although one will run to the bath anyway, and meow or just sit there until it is turned on! So long as area around the drain is clean, and it's only dripping, there shouldn't be a problem.