I have a 98 genIV 454 merc. boat was used exclusively in salt water. I bought it knowing that it overheated in the higher RPMs. I replaced the seawater and engine water pumps. The boat pumps water out the exhaust. I tried a water test and upon runnning at higer rpms the overheating alarm want off and I shut it down. It sounded like the oil was popping and crackling although the actual engine was not that hot. I noticed the oil pressure dropped from an idle pressure of 80 to around 20psi upon shut down. I am looking for any thoughts on this matter.

Have you checked the thermostat?

Give us a little more information like type of boat, drives, external exhaust or thru drive. Are your risers hot (should be able to put your hands on them without burning them)??

How do the spark plugs look? Running lean? Oil temp real hot?

Could have reduced water outflow through the exhaust due to corroded/partially plugged water jackets in the manifolds/risers.

My guess would be salt or corrosion build up in the water passages or maybe the oil cooler.