The best waterproof case is Ikelite. If you watch the big auction site long enough you can find a case that has not had any holes/controls drilled in it. This may cost some $, or you may get one for $10 like i did, it all depends on who else is interested at the time. Use waterproof potentiometers (Vishay is coming out with their P13SM soon, not quite available to the public yet, unless you order a lot). If you use regular pots (or even the p13sm) waterproof them with APMHexseals. Find a IP68 (waterproof) switch to use for the on/off switch, and use Lapp or similar strain reliefs where the headphones and coil cable comes out of the case.
Ikelite is heavy, and can be tricky to drill (use the plastic bits or end mills - slowly), so you may settle for a pelican case (or similar product such as an otter box), or Aqua-pac with cable-clams (as some have posted about).
Don't go with the headhunter pi, it is a great detector, but It is just like your machine, but without the SAT control. Worst case scenario, chest mount your control unit in an aqua-pac with cable clams. Adjust it while knee / waist deep, seal it up, then hunt. That would be more waterproof than the alternative. Do a findmall seach on the products I have mentioned and you will find lots of informative posts.