We've used the Hobie i14t tandem inflatable extensively down here in the
Turks and Caicos. Including numerous trips to other islands, one a six mile trip across open
water. I don't know about 20 knots on the bow in any human powered inflatable, but you might want to check into the Hobie Mirage Drives instead of relying on arm powered
propulsion. We could maintain 3-4 knots easily for hour after hour using the same muscles used to ride a bike. Lifting paddles up into the air doesn't really
work anywhere near as well as those drives. And you have less windage overall, with the added benefit that your hands are free for things like drinking that
beer you just bought. Or taking photos or whatever. Trolling. The Mirage Drives are much more efficient, and you still have your paddle if you want it. We typically just used the paddles for reverse. The Hobies have sail kits, and we bought one for the inflatable, but the best I can say about it is that it works well on anything up to a beam reach but performance drops off rapidly with the
wind on the nose. And they also have a Torqueedo that drops right into the Mirage Drive well. That might be your best answer.
Electric Inflatable.