We lost a couple cats in water tanks when I was a kid. I think we put a board across the top, that way if the cat wanted to drink out of the tank they had a platform to stand on. Then, if they fell in, they could hopefully get their claws up and over that board, grab on and pull themselves out. This only works if your water tank is always full. If it's too low, they will be too far from the platform to grab it with their paws.
We had one cat drown in a plastic rubbermaid tub because I set it next to the fence. Apparently the cat was playing on the 5 rail fence, lost her grip and fell into the barrel. Very sad to find a floating kitty
