Here's some places to look:

First, I would check and clean the strainer at the intake.  That's where I've most often found a blockage problem.

Second (if that doesn't cure it), check the intake seacock.   I've had a few times while underway where it got stuffed up with seaweed (even a floating bit of plastic could get sucked in); close it, undo the hose from it, and then open it (don't worry, there won't be a firehouse of water coming in, its a managable flow) long enough to poke a chopstick or something similar to clear it out.

Third (if the above don't cure), there is the possibility that there is an obstruction in the heat exchanger ("HX").  I've never experienced this, but  have read that sometimes when an impeller fails one or more of the vanes that have broken off travel through the hose to the HX and get lodged there.  So remove the HX, then remove the end caps and flush it out.

On the Great Lakes & Upper Mississippi River we have zebra mussels that clog up intake lines, showing up in late summer.  The gap between my rudder post and boat bottom skeg (???) in front of rudder post gets filled in by mussels and I have to clean this out after haul out.  Have diver use coat hanger to ream out water intake?  I think more likely you may have pices of old impeller clogging heat exchanger.

Had a friend who put in a new impeller but still had over heating. Turned out that the impeller is rubber but there is a ring of metal in the impeller axis hole. The bonding between the impeller and the metal ring was weak and while the ring turned at the normal rpm, the impeller didn't causing it to pump less cooling water. Might want to just try changing the impeler again.