I take it this is a cat you're talking about?! (You didn't say)
Firstly, just find a dish which wider at its base than at the lip. There are plenty on the market, and they can't be tipped or knocked over. If it persists, glue it to a square of wood or a tile.
Secondly, simply don't leave the toilet seat up, EVER - and put a notice by the toilet to tell everybody else to BLOODY CLOSE IT, too, adding '...to avoid cat drowning'. (I don't know why people leave the things up, anyway - there was a piece of film on TV a couple of years back, which was taken with a high-speed camera and strobe lighting - and the amount of fine upspray that came out of the toilet when it was flushed was astonishing, carrying in its wake a large volume of bacteria and viruses). Or if you or your partner are a little clever, you could fix up a foot-operated lever by the toilet bowl, which males would have to put one foot on to keep the lid up when peeing! It would automatically close once the foot is removed.
Thirdly, bath him in the shower - obviously shedding your clothes (but keep some form of skin protection on, if he goes all claws). You will need to ensure that he cannot push the door of the shower open. Once a cat is thoroughly wet, the struggling usually relents, as they've lost the battle to stay dry.
You could also try putting his water dish inside one of those litter trays with a detachable overlip (not sure if that's the term, but I think you'll figure that one out!)
Cats are mostly not stupid, and usually grow out of stuff that makes no sense.