I was working on another of my hobbies the other day (homebrewing beer) and I realized I have some pressure test tanks sitting around my house that I can use to test stuff in.
They can't be used for a full ROV because they're too small. But if you're like me and some of the parts of your setup can be tested one at a time (I need to test my wire entry and also endcaps on my thrusters at depth) you can use something like this and it's pretty cheap. The opening at the top is an oval opening with the longer side about 5" and the narrower distance about 3" (I can take more accurate measurements if anyone is interested).
You can get the cornelius kegs used for about $50.
I'm going to put parts in there, fill it with water and then apply air pressure from a compressor. They can handle about 100psi safely which would simulate about 200' (or 60m). If it gets too much, there is a pressure relief valve that will trip. I think I might even 'hot wire' a thruster and enclosure and put them in there with the power on just to see how they function under full pressure.
That's about twice the depth I plan to use so I think it's a good way to test components out before 'going deep'.
Here's a pic of one.
