You may want to enable "Allow Freelook" in Gameplay setting. Some of under-water fight is deep and Freelook help you swim better.
Because there are some enemies in under but the game's auto aim cannot aim at them when enemies are exceed a slope of around 58 degrees. Beside that, I found no way to swim down without enable Freelook. Even with "Courch" button didnt do a thing. So be sure to enable Allow Freelook in Gameplay Settings.
