Hmm, I bet you could just put a prop on the end of a bowden cable and tow it behind a boat, without having to worry about waterproofing the dynamo. Point the axle down and let the cable flex to stop it rising above the surface.
@neftaly I've seen logs that work like this just using a string, a tortionally stiffer cable probably way better though. Unfortunately the max water speed is 6.8 miles (assuming statute miles, not nautical) that is 6.04 knots. It's not too hard to hit 6 knots so I'm worried you'd damage this towing it behind a sailboat.
This device is clearly optimised for air.
In water you would use a log type screw to shed weed and debris and avoid damage.