I'm trying to keep the OP's use in mind here.
He has a fishing boat. It's not a ski boat, nor a drag boat, nor an off-shore racer here.
His operating patterns are likely this: start up and idle while preparing for launch, idle out of zone, full throttle to get up on plane, back off to (around) 2/3 throttle to cruise to a fishing spot, idle down to not scare the fish when getting close, then stop. Repeat as necessary. I aksed him in another post if he's doing a lot of trolling and the answer was "no", so excessive idling (fuel dilution) is not an issue.
With this in mind, he's likely to treat his motor no different than the engine in my wife's minivan. Start up and wait for the kids to get in and buckeled up, idle down the long drive to the county road, pull out and hammer it until up to 60 mph, drive to school for drop off, then "city traffic" to work.
Sound familiar? Yes, a boat uses more fuel per distance traveled, but the useage pattern is VERY similar.
This boat engine needs a 195 deg T-stat, and some HDEO multi-weight, with an oil-temp conditioning device (block water to oil heat exchanger?) to bring the oil up to temp and then hold it there as soon as possible. Then multiple UOAs.
I'm beginning to understand why conventional vehicle engines that are converted to marine use are subject to high wear. They probably operate too cold, and use oil that's too thick. The outside ambient air temp has very little effect on the motor. Typically, a boat motor such as this uses the lake/river water as a direct cooling medium; there is no secondary cooling system. On a houseboat, with a diesel gen-set, the lake water is drawn in to cool a self contained system that cools the engine; a water-to-water system. But most boat motors, be they vehicle based engines or marine-specific two-stroke engines, use direct water exchange. Running a 351 Windsor engine at 165 deg F with a straight 40 weight is going to definitely produce some high wear metals.
It seems apparent to me that this motor is a glorified truck engine in both origin and useage pattern, and should be treated as such.