Boat overheating issue help

I just picked up a new to me boat, 04 24v, with the 375hp motor(w/ closed cooling). The issue I am having is the boat is overheating. I took it out for the first time and we drove it out of the marina everything was final got it up to 2500 rpm's and regular temp then all of a sudden it spiked. So I slowed down and it was still spiked. I shut her off for 10 min or so and went to see if it somehow fixed itself....didn't happen. i thought must have been impeller(no prob),we got back to the marina loaded it up and off to the shop. My shop guy told me the impeller had an tiny imperfection in it so he swapped it out. Then tested it and realized the boat was super low on antifreeze. So he filled that up and still having the same problem. He told me the lake water is getting to the heat exchanger, so the intake is staying cool but the outflow is super hot. He also said there is minimal water pumping out of the impeller pump at idle. I just thought I would post here and see if anyone has had similar issues and what they did. Thanks!

Assuming you have a marine power 6.0? If so, those particular motors are a bitch to "burp" to get all the air out of the block on the closed cooling side. You need to let it run at a high idle, at temp, and crack the hoses at the thermostat housing to release any air pockets.

And / or pop the end caps off the heat exchanger and flush out the passages, debris can accumulate in their and not flow enough raw water at higher engine speeds to cool the antifreeze properly.

Thanks Meat. I will tell the mechanic, He is ordering a new pump and gonna see if that will do the trick. He has good flow out of the pump at high rpm, but when idling he's barely got anything.

Your "mechanic" is in over his head..now throwing parts at it...aka your money....take it to a shop familiar with your brand of marine engine.

Take a look at your hose connections. I bet you have a air leak on the vacuum (intake) side of the pump. If you have a strainer, start there.