I'll pipe in on this one, too. As Mike said, check the pressure relief valves first. That would be my first guess.
If I were facing your problem, I would run the boat with the hood off and have a friend drive the boat and I would use a heat gun to confirm that the manifolds are truly over heating - in excess of 180 F.
Your OEM t-stats will start to open around 140 F and should be completely open at 160 F. Full open is only 3 mm. You may want to test them in a pot of water on a burner with an accurate thermostat. It's not unheard of that t-stats get out of specs. I would be reluctant to replace those t-stats with cooler ones. The engine was designed to operate at specific temperatures. Anything significantly cooler could cause excessive wear and end up contaminating your oil with gasoline.
If you are operating in shallow areas it is possible that you may have ingested sand or mud into the water pump and other passages. If the pressure relief valve cleaning doesn't fix it, I would be prepared to replace the water pump - not just the impeller, but the entire pump. It's not that much more expensive than the impeller kit. Over time, the impeller will wear the sides of the pump wall, causing a bad seal with the impeller vanes. At higher speeds problems with a worn water pump seal will more likely show up. When doing that, and with the bottom unit dropped, I would thoroughly back flush the water passages using a garden hose stuck into each t-stat opening and sealed with a rag.
One other observation - at 4200 rpm or around that number, is when the V-TEC (variable timing) operation kicks in. It may be far fetched that the failure of the V-TEC operation would cause an overheat alarm, it is worth noting of all of the above remedies do not fix the problem.
A final piece of advice - throw that Seloc manual in the garbage can and invest in the official Helm Shop manual. It's about $116 on Amazon or eBay. But they are worth every penny based just on the detailed maintenance chapter (3) and the V-TEC diagnostic chapter (5). It will save you many $$$ in the long run.