This may sound crazy. How about a quadcopter designed for underwater use. Brushless motors operate perfectly well submerged and the rest can be waterproofed quite easily. Idea being that one would use small props, basically the quad would operate in a different medium but essentially it should still stabilise as it does in air. I thought about escs (which don't like water) getting hot but they could be exposed, covered with tectyl or some other waterproofing spray.
As a fail-safe, waterquad would be slightly buoyant so in case of power loss, it would merely float to the surface. One would then have keep throttle on all the time to stay submerged and power up to sink as opposed to normal operation. I know that 2.4ghz does not work underwater but the older 35mhz systems do. For fpv, 1.3ghz could be used ?? Also I wonder which flight controller would work best, kk2 maybe for cost-effectiveness.
Has this been attempted before and if so, any success? Couldn't seem to find anything on this concept. I'm itching to start building but somebody please stop me if I'm wasting my time.