I love the Nik IV, which tends to be very affordable. So far I have not had much trouble zone focusing with it underwater. The theory I have heard is that your underestimation of distance due to maginification offsets the focus shift. If you are not used to zone focusing it can make you nervous. Got some decent pictures with it snorkeling last summer, which I would post if I hadn't had to do the color correction digitally (the water was brownish, which I had not expected). Haven't been deeper with it yet, due to a series of problems that have kept me from diving. I don't miss being able to set shutter speeds under water, as I can usually make a decent guess as to when it is getting low. At snorkeling depths - 400 ISO, f/8, and be there. Deeper, use flash.

The earlier Niks are smaller, lighter, and more able to endure damage due to the absence of electronics.

There were also some cheaper medium format underwater cameras back in the 50s, though they tend to be harder to find. They are basically just underwater Brownies, nothing fancy.