Maxum did not use Volvo only Mercruiser, what outdrive is on the boat? Volvo and Mercruiser both use GM blocks so swapping is a none issue but I suspect the outdrive is still a Mercruiser Alpha.
A friend of mine had the same expierence as you and a year latter lost the engine due to this overheating expierence. I donít trust the helm gauge much and use a handheld IR temp sensor to take readings at the first trip. I also replace the impeller every two years. The helm gauge only provides the engine temp and not the exhaust components.
Best of luck.
1997 2400 SCR
5.7 Vortec / Bravo 2