‘90 Mirage Merc 454 Magnum
Changed thermostat twice, first 160 degree then with a 142 degree. New temp sending unit, new sea water pump. Still getting hot. I don’t get it, please help!

Did old seawater pump come apart? Debri plugging system?(coolers) Engine circulating pump ever been replaced? Bravoitis?
Disconnect hoses and back flush system with water hose. Visually inspect cooler for debris/blockage.
I guess first thing, are you sure it is getting hot and not just a bad gauge reading?

I have backflushed, inspected coolers, did not do the circulation pump as I am getting good flow exiting the exhaust, seawater pump did not come apart but did have some scarring on the fins and I was not getting water out of the exhaust. I am new to you he game so not sure what bravoitis is, I’m guessing something that’s commonly wrong with bravo engines?

Bravoitis:
https://www.google.com/search?q=brav...&client=safari

btw: if there is any scarring/scoring/grooving that the the impeller sides and tips will go against, this can cause eater flow and psi issues. Therefore, Sometimes more than replacing just the impeller is needed.

Before buying any more parts I would get an IR temp gun and verify gauge readings. They are inexpensive at most any hardware store.

Thanks I will try that, where is a good spot to aim the ir gun to get a good temp reading?

If it had bravoitis, wouldn’t I not be getting good flow exiting the exhaust?

Shoot the temp sending unit and several places around thermostat housing to see how it compares to gauge reading. Then try to follow the water flow : inlet hose -through coolers-to stat housing- to circulating pump. From stat housing to manifolds . See if you find a hot spot indicating restriction.
I think Bravoitis shows up at higher RPM. Pulls enough water at idle to stay cool, but at higher load doesn't have enough volume to keep engine cool.

Connecting a water press gauge to block drain (via hose) might help determine if water is having trouble getting in or out.

Another "trick" is to temporarily install clear hose in place of various black water hoses and see if air is getting pulled in at higher rpms.

Is this boat new to you? Can it get to 4600 rpm? If it's over propped that can contribute to heat. Too much timing can be a part of it too.