There are a few underwater point & shoot cameras that are fairly inexpensive but may be very limiting.
I believe Nikon does or did have a dedicated underwater camera that was supposed to be pretty good.
There are water tight cases that are made specifically for some camera models. Most likely made for Canon or Nikon.
There are also some less expensive options. I have seen underwater camera bags. They are basically a plastic bag with a "lens" to shoot through.
Try a google search.