check to make sure that there is an adequate amount of water making it through the system. An easy way to do this is to use a "fake lake", the plunger looking thing you connect a hose to and shove up under the boat while it is on the trailer. At 1000 RPM or so it should be taking pretty much what a garden hose can deliver.
At idle, you will probably notice that the water builds up inside the engine/exhaust for a few seconds and then it will blow out a large surge. At higher RPMs you should be getting a pretty good steady spray of water coming out the exhaust.
if the volume seems low, then either the impeller is bad, there is a blockage, or a leak.
A leak between the raw water pump and the through hull will allow the pump to suck air instead of water. Check to make sure that the hose is not cracked and is hose clamped tight at both ends. A previous boat that I owned had a flushing attachment that if the valve was left open it would suck air instead of water.
A blockage can occur in a lot of places. I had mine block one time on the intake, at the oil cooler for the transmission. The path was the thru hull, then to the oil cooler, then to the water pump. The passages in the oil cooler were far smaller than the "grill" on the thru hull and that is were a lot of weeds managed to lodge. Check to see if there is a valve on the thru hull that might be partially closed. It is also possible that there is a blockage downstream of the pump. A stuck thermostat could cause this problem. I have heard of cases were the impeller breaks apart and pieces get washed into the engine, where they lodge into small spaces and cause problems.
Impellers will wear out, but not in 20 hours. Unless you have run it dry. Running the engine without pumping water can melt the impeller in less than a minute.
If there is an adequate amount of water getting through the engine then there is a different sort of problem. What do you know about this boat's history? I have seen a cracked head or leaking head gasket cause overheating problems.