This is not really an option but I read just yesterday in the Vetus catalogue of a water cooled electric motor they produce which powered a 6 metre launch (if I remember the length correctly) at 6 m.p.h. (note not knots). This I consider a reasonable performance but, as I say, it won't help you as it is an inboard. However, I think it does indicate that electric motors are improving and, under the right conditions, definately have a place on certain boats.
By the way, although Vetus did not give a price, they claim the electric motor costs less to purchase than a conventional inboard.
The set up showed in the catalogue interestingly can also be rigged as a hybrid with the electric motor charging the batteries when the main engine was running.
This obviously does not solve your problem but, given what I learned yesterday about the electric inboard, I would not rule out electric propulsion until I examined the marked more carefully. After all, the thought of silently gliding through the water does appeal to yachtsmen!