Otherwise, few boaters run enough fuel through diesel engines for fuel savings to make up for the high initial cost. Not only is there the issue of weight, but the water resistance on a larger hull. Gas engines begin to build up too much internal heat and the strain begins to result in lower service life in larger boats. V-8's that are the largest gas engines available. These engines at 300-340 hp usually do very well so long as they're not pushing a too heavy load.
Internal displacement is the best measure of an engine's ability to deliver power efficiently.
The inviolable rule for service life is that the more power is squeezed from an engine block, the shorter it's life span. A 350 CID block generating 260 HP is going to last a whole lot longer than the same block putting out 350 HP, whether its gas or diesel. Yet other factors come into play, engine speed and weight. There is no escape from the fact that fast turning diesels have substantially shorter life spans. Slow speed diesels can be longer lived precisely because they do turn much slower.