This is a very nice drinking pond for cats. I have it outdoors, but mine doesn't leak anything at all, so I could have it indoors. If it had leaked I would have put some coconut oil or something to seal it or tried with milk or sourmilk once... as you would have done with ceramics - to seal it.
Update: The pond started to leak after a few days, so now I tried to seal it with coconut oil that I melted and brushed on the inside. We seldom have more than 24 degrees outdoors, so I think it should be ok? Coconut oil doesn't seem to be toxic in any way for the cat and the amount leaking to the water should be miniscule.
4 bottom layer, 4 top layers, 4 side layers