The prototype in the linked video was first tested back in 2023, and since then a few startups have set their sights on the technology (e.g. Nomadic Drones and Voltair).
When working on the linked prototype we ran into some fundamental issues. Firstly, the recharging will only work with AC lines, and there's currently a lot of hype around UHVDC lines which are not compatible. Secondly, the AC lines must carry substantial current (ideally thousands of amps) for the recharging to not take forever. You can of course carry a much larger transformer on the drone to compensate, but this will in turn severely limit your flight time (ours was 1kg on a drone of 4.5kg and we could charge with 50W from 300A line current). You also have to account for significant daily fluctuations in the line current. It'll be interesting to see how the tech evolves, and I'll definitely be following these startups closely.
In their report (https://gitlab.com/hybrid-drone/paper) the authors mention that they implemented one controller for flying and another for diving. The transition seems to be initiated manually, but they propose a few ways of automatic detection and transition.
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.