OK, #1 with a boat your oil pan gets no air flow unlike a car. Get yourself a bigger oil\water cooler. #2 For a boat your RPM is on the low side. @3000 RPM your lugging the motors and that will bring the oil temp up big time. Your motor temp wont show this because its not a closed cooling system.
I do know alittle something about this. At this time I am a fulltime student in a mercury marine program 8 hours a day five days a week.