Here’s a really cool underwater ROV project that has detailed documentation on how to build one yourself.
The ROV submerging technique was originally using part of a bicycle inner tube and a car windscreen washer pump. However this method was very slow to respond and difficult to control and ended up either dropping the ROV like a stone to the bottom or rising to the top. Probably a little worrying having water pumped into the same chamber as the electronics but I tried to make it as reliable as possible and with this in mind I incorporated a leak detection unit which alarms on the GUI and automatically surfaces the ROV. I gave up on this submerging method when pumping water inside caused excessive internal pressure and blew off the end cap on the ROV despite being held in place with two clamps.