Another thing I've thought of doing is installing a Sword Marine JetPac diesel jet.
JetPac photos
The 150hp model is rated as propulsion hp, is actually positively bouyant over it's own weight (150 lbs positive). The drawback is the cost. I understand $18K installed with all gauges and controls. The 200hp model is supposed to be about $20K.
Durability is good. I understand the US Coast guard is currently evaluating these for use in small RIB tenders because of the short engine life they get from outboards with young coasties at the throttle. Whether they buy them or not remains to be seen. The JetPac is a factory option from Key West boats on their 19 & 22 center consoles.
I do know that the JetPac diesel won the NMMA 2003 Innovation Award for engine and propulsion systems.
I'm not sure I'm ready to pay the cost, however.
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Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury