It turns out that you get a tidal power electricity generator.
The Faroe Islands has been using the Minesto Dragon for the last couple of years, but I only came across it last night.
Interesting system. It tethers a marine glider and lets it fly in figure 8s across the tidestream. That gives it enough speed to run a turbine at the back of the glider in currents as slow as 0.5m/s, generating up to 1.2MW.
This is the best video I've found



Only obvious problem I can see beyond corrosion is that it would be a real whale hazard, unlike offshore wind turbines

The amount of energy that could be harnessed from tidal and wave power is huge, world changing! Also with tides being predictable and reliable that would alleviate some of the intermittence that is a problem with wind and solar.

I think the main reason we don’t see a lot more tide or wave power is because it’s difficult and expensive to install and maintain. Machinery and electronics underwater have a ton of things that could go wrong with them