The watery sphere spell description says:
You conjure up a sphere of water with a 5-foot radius at a point you can see within range. [...]
Any creature in the sphere’s space must make a Strength saving throw. On a successful save, a creature is ejected from that space to the nearest unoccupied space of the creature’s choice outside the sphere. [...] On a failed save, a creature is restrained by the sphere and is engulfed by the water. [...]
Does that mean the creature starts to suffocate?
The water created seems to have the properties of normal water:
When the spell ends, the sphere falls to the ground and extinguishes all normal flames within 30 feet.
I know that "spells only do what they say they do", but since it can be derived by physics that fire spells emit heat, then watery sphere should also prevent breathing. Using similar reasoning, there's no effect that allows the creature to breathe inside the water.
I'm a little concerned when I think that the watery sphere and hold person combo can kill a single powerful creature with ease; it doesn't seem right.