This is due to the poor propagation of radio waves underwater. [Simon] wanted to build such a drone, but elected to go for an alternative design with less strings attached, so to speak.

Building an underwater remotely operated vehicle (ROV) is always a challenge ... the CPS 5 has five brushless drone motors with 3D printed propellers, which are inherently unaffected by water ...

Deep Drone 8000 is a 4,100 pound Remotely Operated ... The operator can control the ROV in all six degrees of motion with auto-control functions for depth, altitude, and heading.