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