Any ideas how the signal is transmitted once the drone is underwater? Typical RC transmitters run 2.4 GHz. Some systems use 700 MHz. The former wouldn't penetrate at all, the latter might penetrate to a shallow depth, but I think that what's shown in the video is deep enough that the signal would get attenuated. There doesn't seem to be a tether and the drone doesn't seem to have any computer vision capabilities to fly itself whilst under water. I wonder how they solved this.

As far as I can tell from their report (https://gitlab.com/hybrid-drone/paper), the authors did not consider underwater RF transmissions in their prototype design and simply use standard COTS components typically used for DIY UAVs.

IMU that determines rotation rates and acceleration without vision. A drone can fly and hold attitude with some accuracy without any outside help, it doesn’t need cameras or outside control. Only GPS to counteract drifting but that can be neglected in the very short term.