Thanks Ron, Joel and Mike for your advice.
My previous inboard had a FWC system and this worked really well. This would be my preference as I am sure the engine will last longer when used in salt water. I will have to live with RWC for now as I have used up all my wife's allocated boat spend budget!
I read your thread on flushing boats on the trailer and the possibility of causing damage by pressurizing the system with a directly connected garden hose. I usually disconnect one hose from the strainer and shove the garden hose in there, this doesn't pressurize anything directly and you can see when you are sucking more than the garden supply. I run this for 10 - 15 mins.
What are you referring to when you say "install a fresh water flush"? There seem to be a number of options but the ones I have looked at seem to block off the intake and look like they end up pressurizing the system from the attached garden hose.
The overheating issue I have is really strange. Most of the time I have no issues but when doing something like teaching someone to ski, where there are bursts of power applied and then a quick drop back to idle multiple times in a row, I will suddenly see the temperature rise rapidly to the max and the engine warning light comes on. No amount of increasing revs in neutral helps. The only thing that brings the temp down is to get the boat on the plane. After a few seconds, the temp rapidly reduces to normal. When the temp is at max, the engine doesn't appear hot. My previous boats, you would get a feel that it was running hot by lifting the cover. The combination of smell and heat usually gave some indication that it was running hot. Doesn't seem to be the case here, although my "scientific method" for cross checking engine temp may not be valid.
I have checked all hose connections from inlet to pump and all looks ok. This weekend I will take apart the raw water pump and check the impeller and cover gasket, although the guy I bought the boat from said he replaced the impeller which was only a few weeks ago.
This is really worrying me as I seized the engine in my last boat through similar issues with temp and oil supply. I almost get the feeling that the water is boiling in the top of the engine near the temp sensor and requires high water flow through the engine to push this out. I sometimes see what looks like steam near the exhausts when having these issues. This would be my only explanation for the temp showing max but engine not appearing to be overheating.
Anyway those are just my humble opinions but keen to hear from the experts.