Very interesting: only one moving part, aside perhaps from steering surfaces. Or is it passively stable? This sidesteps Makani Power's absurd complexity, able to use buoyancy for loft, instead of needing to be a self-piloting powered craft. It would be good if the figure-8 path could be a natural result of tether and vehicle geometry.

I wonder if the tether could carry air to establish positive pressure in the vehicle body, against leaks. Buoyancy from the sea floor should provide all the pressure needed.