Dan have you seen these?
Click on the "Technology" link. Apart from the whacking great wind generator on the top of the mast, I'm warming to the idea! They actually have their electric motors submerged - so they must be confident in their sealing arrangements, and 5 hours at 6 knots sounds like a half-decent endurance.
Submerged motors are old hat, just look at all the deep diving submersibles manned and unmanned there are arround the world working every day, and have been since I worked with them in the 60s. Mind you 5 hours endurance is pretty poor compared to the ten times that my boat has.