Moving camera into underwater

I have a simple move camera from sky into water.
But I can not figure out what to use to do the underwater effect once the camera is in the cube of water.

is the water fluid? if so you could place an un-rendered cube arround the camera, parent it to th cam and set the cube as a fluidsim obstacle

Well the Sim is not yet needed. It’s just the under water look. But to get the entry of the water keyed at the time of water entry.

So from a sunny sky view to a darker murky view of under water. Just dropping into a cube does not work due to the cube not having any depth. And timing the camera in front of a cube sounds best, cept for that gives a tinted alpha look with no real depth of water dof.

Hmmmmm Voxels comes to mind. But what happened before voxels?

After thinking about it some more.
If there was a cube on the camera. But the material would only turn on if it touched another material. AND only showed that materials cube portion viaa a mask that was created dynamically via the amount that it was dipped in the other material.

Think, ice cream bar and chocolate coating.