I would go through a systematic diagnosis to find your problem. I doubt if the boat overheated when new. If you are overheating at 2300/2400 and WOT is 3000 then you have a significant problem.

First measure the water flow out of the exhaust by catching it in a bucket. Compare against spec. If you have a problem finding the spec, PM me and I will look up the spec for a 27 hp Yanmar, should be about the same.

If it is not within 25% of spec then try the same thing by removing the hose from the R/W pump. You can run the engine for a few minutes when cold to check this out. If the R/W is within spec then you have a restriction in your R/W cooler, or exhaust elbow.

If still not within spec then you have a problem with the pump or the suction hose. I have seen collapsed hoses and even a mussel that grew up inside the suction fitting cause overheating.

David