Why do you care?
You are touching a physics body when you collide with water, normally.
It should be enough to give you the type of response you need (for audio queues and what not, its all cpu side anyway).
The reason why, is probably actually just that:
You aren’t colliding with anything.
Water is on the shader/gfx.
Physics is on the CPU.
Ergo there is no response or communication at all between one and the other - unless you make one of some sort.