Pedro

I have never done underwater pinhole, but I have done both pinhole stuff and underwater stuff. A few thoughts......

Pinhole exposures are long, moving water makes most things underwater move, this is a bad combination. You will need to hold the camera steady, which means on the bottom, and you will probably want a subject which is on the bottom.

One photo that I got once underwater which was interesting, and something similar might be possible with a pinhole....

Set up your pinhole as a wide angle - then put the camera on the bottom, pointing up for the exposure. If the water is calm, the center circle of the image will be looking up at the sky and you will see things on the shore - outside a certain angle the light from outside is reflected completely, and what you see is a reflection of the bottom - so the result is an image with a circle in the middle with sky and some shore features, and around the outside, detail of the ocean bottom. Trick is to find a combination of very still water and very shallow.