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