In turbodiesel engines the primary difference in horsepower ratings from land to water is that you have an unlimited heat sink with a boat. Regardless if the engine uses sea water or a closed loop system. The components are typically the same except for a few key components, the exhaust headers, the turbo, and most importantly the water to air cooler for the forced induction system. The exhaust components are water jacketed for safety purposes to help avoid fires, this feature, as expected, reduces horsepower. The jacket coolers remove energy that would normally go to the turbo and release it wastefully to the lake/ocean/bathtub what have you. The internal temperatures are the same as component materials limit of combustion chamber temperatures. The major difference is that an unlimited water supply to the inter-cooler can drastically reduce the charge temperature compared to an air to air inter-cooler on a road vehicle.
Typically in multiple engine configurations they rotate opposite each other. In single engine typically standard rotation, because many components are the same as land engines, therefore cheaper to manufacture.
As far as naturally aspirated, I could speculate on why gas boat engines have higher horsepower ratings than land engines. Considering boats relative weight, completely different load demands and cycles compared to vehicles, and that horsepower is time function of torque, well I could guess anything, but I have limited experience in that department and will digress.
i'm just as curious as many these other guys to see the end results!
