On our 2004 Hunter (Yanmar 2GM20F) the only engine overheating I've experienced turned out to be because a vane had broken off the impeller and was partially obstructing the flow. When I noticed it was gone, I poked around, but basically I was too lazy: so at first, I just replaced the impeller. After one particularly scary time (the engine temp alarm went off in a high wind, high worry situation), I decided to pull all the hoses between the water pump and the heat exchanger. I found the chunk of rubber in the end-cap of the heat exchanger and removed it. No problem since then.
I believe you said it's a new-to-you boat, so maybe the PO had an impeller break up before you got it? It's pretty easy to check the heat exchanger.