The control water spell has the option to separate water and form a trench on a cubic area of up to 100 feet per side:
Until the spell ends, you control any freestanding water inside an area you choose that is a cube up to 100 feet on a side. You can choose from any of the following effects when you cast this spell. As an action on your turn, you can repeat the same effect or choose a different one.
Part Water. You cause water in the area to move apart and create a trench. The trench extends across the spell's area, and the separated water forms a wall to either side. The trench remains until the spell ends or you choose a different effect. The water then slowly fills in the trench over the course of the next round until the normal water level is restored.
This is easy to imagine when cast at the water's surface. There is air above the water, and so parting the water forms a cube-shaped trench with breathable air.
What happens if the targeted area is all underwater, not near the surface? Does parting the water create a cube of air?