I believe inrunners is better suited for boats, because they are much better subjects to water cooling.
Plus, the inherent geometry of an inrunner motor lets it put down more power and efficiency than a similarly sized outrunner.

However: Lady luck has it so that there is a series of cheap outrunners that is supposedly quite good, even compared to expensive ones. I never tried these motors myself in boats, but I got tips from more experienced guys than myself that these motors is good.
The only reason these outrunners is competative in budget racing though, is because there is not to my knowledge any low priced but good inrunners.
However, put a quality inrunner up against a similary sized quality outrunner, and there is no doubt. The inrunner will make mints meat of the outrunner.

These cheap outrunners in question is some Turnigy heli motors.
I tried the 450 in my E-sky Belt-cp helicopter. I didnt notice any significant difference over the stock 450 motor.
I believe the difference shows when you push them hard. Since these ones got cooling fans, I believe they are able to push it farther before running hot. Overheating coils is the weak link for outrunners, so it stands to reason.

http://hobbyking.com/hobbyking/store...idProduct=9258