Trolls are fast on land, they can walk much longer in water than you, since they're way taller. I've never seen a troll starting to swim, and I doubt they will start diving.

And skeletons are just bones. They don't pose much resistance to water. So let them run on the ground. Can't get you anyway. It's more fun than anything threatening.

Not sure what you mean with "ogres". There are no ogres in the game.