LOL Yep, I know it's tricky... my 3rd baby/toddler is very active in the bath. You could try giving toys etc to keep him seated but my experience is that is doesn't really do much. Mine is at the stage of chucking them all out onto the bathroom floor then wanting to go get them.... over and over again

All you can do is sit there beside them and watch them I think. Mine have slipped and even slid under the water. They just emerge coughing. They won't drown if you are there. A child has to be under for more than a few seconds to drown. The biggest worry is that they hit their heads. If you have rounded edges etc this help. We have a sharpish tiled edge so I have to make sure I sit right near that.

Try not to stress, it's a phase...

you could try those bath paint things that you can "paint" on the sides of the bath? But even that will lose the novelty factor. hopefully someone will have a better idea? i'll be watching this thread!