1994 Evinrude 90 HP couple of issues

Good Morning All,
I Purchased this boat from someone who had it for a year and really knew nothing about it. It has a 90HP Evinrude on it. E90tlerc. I was looking it over because it does not like to run at idle, give it a little gas and and runs great! I checked the compression on it last night and I got cylinders 1-3 at about ~89 psi, and cylinder 4 at about 80 psi. Now granted I am using a harbor freight gauge so I don't know if that is why these seem low. After doing the compression test I let the motor run a little bit at very low RPMs, and then got a constant alarm going off. I believe this is overheating? if so is there a good way to diagnose if its water pump, thermostat or blockage? Or does it just make sense to replace them any way for piece of mind?
Thanks

Some will argue.------" a new to you outboard " needs a new impeller.-----Those compression numbers along with the history of these motors is a concern.-------I say pull at least one cylinder head to inspect the cylinder walls.-----If nothing is wrong a new head gasket is the same as 15 minutes shop time at a dealer !!!

When you say cylinder head are you talking about just the cylinder head cover, or pulling the head completely?

The cover allows you to look at the water jacket.------The 10 bigger bolts allow you to remove the cylinder head to look at the working parts of the motor.