Most of my photography is underwater.
I use a Panasonic GX7. This is a micro four-thirds format camera. It has interchangeable lenses like an SLR but it doesn’t have a mirror and is considerably smaller.
It lives in a Nauticam housing which is fitted with a leak detector. Hopefully this will give enough
warning to prevent the camera drowning if the housing does leak.
The most useful lenses for underwater work are a 45mm macro and a 12-35mm zoom.
I also have an 8mm fisheye, a 20mm prime and a 45-200 telephoto.
You can see some of my images in the Gallery.