My only experimenting with this was to get some glass cut and silicon glued together what looked like a tall fish tank with a small base.
At the bottom of it I placed a wooden false floor with legs to create a space underneath that I could put rocks in to act as ballast.
The camera was screwed to the floor with a 1/4 Whitworth cap screw.
The top of this glass container was open, so the camera was submerged but not the container.
Shutter was via cable release.
It was a manual focus camera so that was all done with a predetermined focus distance.
It was also a medium format film camera so I just used the waist level view finder (except at knee height) for framing.
The design worked fine to get some pictures of salmon in their spawning grounds.
Focus was the hard part.
I used it once maybe twice and that was it.
Sounds a bit crazy, but then so does putting your camera underwater in a fancy zip lock bag.
If you are entertaining this idea, you could work something out with an assistant and a tethered laptop to have some control over what the camera is filming.