How do I combat enemies in the water?

The only thing you can use in water is your crossbow

You can't use signs underwater, but if you use Quen before you enter the water, the effect will stay and you can deflect one hit. With the "Exploding Shield" ability it will also knock back the attacking enemies.

This is how I dealt with underwater enemies that were close to treasure, to have enough time to grab the loot and get out of there.

You can only use your crossbow to "fight" underwater.

The crossbow can kill them with one hit, or throw them off and make them go away, depends on the game's difficulty level.

Be reminded that the crossbow doesn't always kill one-shot.

More than once I've tried killing drowners underwater at lvl. 37 and I was lvl. 40 and they died with 2 hits. I've experienced this in Skellige as well as Toussaint from the Blood & Wine DLC.