How do I combat enemies in the water?

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.

Yes you can use the crossbow and shoot them underwater.

The problem for me is I thought that I would one-shot kill them underwater but it only took out around half of their health... Also Geralt takes too damn long to reload another arrow... I was level 3 and the drowner was level 10. And I was playing on easy mode. And I got killed when I wanted to get the spoils of war.

I just killed the under water creatures by using the cross bow under water.

The way to do it, is to swim out just enough for them to notice you, then shoot the cross bow directly at them once or twice to get their attention.

Then swim towards the shore while they chase you, go onto the land to escape and then go right back in the water and repeat the process until they're dead.