Cape cod marine 26 cc or I guess it is now the 26 albin cape cod just did one up w/ the volvo dou w/ a 300 hp boat does 50 and burns just 6 gph. That is a great ride. Yanmar also has a 270 w/ a merc 3 and it's a great package. I just sold a cc w/ twin 200 hp yammies and if I were to buy another i would lean toward the i/o w/ a jack shaft moves the weight forward and you have the reliability of the new diesels.
Think about it Charlie 5 to 6 gph at 50 mph