Good point about the battery charging operation (didn't even think of that). I don't have any guess as to what number of hours are spent in that type of low load condition, but I suspect that most hours on an engine for the average cruiser would be spent used for propulsion ( I could be wrong I often am). If the engine has been proped correctly the load condtions would be fairly simalar to a genset (constant rpm fairly high in the power curve). I'm sure that the racing crowd isn't using their engines like cruisers, but they wouldn't be as concerned with the high number of engine hours because their engines aren't being used for the extensive amount of motoring that many cruisers do. Also, not many cruisers are going to motor their boat at speeds much lower than hull speed (unless manouvering).
As for operating conditions. The marine enviorenment is a harsh place to operate, but if the engine is freshwater cooled with a heat exchanger and is properly enclosed it shouldn't have as signifigant an impact.
Just my .02