Many different things….
But a few questions to help us narrow things down…
1) are you getting good water flow out of the exhaust? Lots of potential causes, and this would certainly cause overheating.
2) Do you have AF in the heat exchanger? Is there some AF in the overflow tank? I bought a boat and was motoring it home…a30 minutes into bashing. seas on Lake Michigan, engine alarm went off….engine overheated. Nursed her into the closest port, and found nothing in the HX. So I know this can cause Heating issues.
3) is the bottom and prop fairly clean? Overloaded ngine could run hot.
4) Any smoke, oil, or other stuff in the exhaust? Something mechanically gone wrong in the motor?
Give us a little more and we can narrow things down for you.
Greg