My cats have done the same thing. They love toilets and if I leave the lid on, they mosey into the restroom and drink water; this has never bothered me much, however. I find it cute.
But if you want to avoid this, simply keep the bathroom door close and the lid always down.
My cats also love jumping on the sink, mostly to drink water. Again, this doesn't bother me either, but if you don't like it, simply make sure to have FRESH water near them at all times. Some cats also like the sound of water; its very rare but I had one cat who LOVE watching the water run in our bathroom sink.
Also, cats can get lonely very quickly and whenever I leave any of them alone and go to the bathroom, for instance, they tend to follow me and cry outside the door. It's kind of cute, but also sad. I usually try to put them with other cats or another person in the house while I shower or use the bathroom.