Not the usual culprits this time on my 1999 Checkmate 28ft with a single 502MPI. Replaced the raw water pump, thermostat and engine circulating pump, Not a quick overheat. It takes a while to creep up to 165 as thats the thermostat setting. It just continues its slow move past 180 and when it gets to 210 before I shut it down and let it return to idle. Within seconds it returns back to the 165 normal temp. I start this slow 5-10 minute process over again each time. Its like its not getting quite enough cooling volume as it gets to a working load at 3000 to 4000 rpm. I know its vague but all help is appreciated.
Thanks
Scott

Being in FL its a saltwater boat and guessing it has a Bravo out drive.
If it will idle all day and not overheat, then check for Bravoits

Bravo one yes. Bravoits?

When the line coming from the drive to the transom plate becomes restricted. The salt causes the hose to swell and gets closed off. The new transom assemblies have a different design. This can also be retrofitted. To verify, need to remove the fitting that the water pump hoses is connected to at the transom

Can look like this



This is the fix
https://forums.iboats.com/threads/br...is-fix.680176/

Is the alarm sounding when it gets past 200* ?? The alarm should sound at 195ish.

Bravoitis, oil cooler restrictions and manifold restrictions is where I’d start here. Raw water impellers can burn easily too. Double check that.

start back flushing the coolers,start w/ the one closest in route to the raw water pump.

I had similar symptoms, turns out it was scale in the water jackets And exchangers acting as an insulator. just didn’t quite get enough heat transfer to keep cool. Cooled right back down at idle.

suggestions above are good ideas also