Re: overheat alarm
In addition to replacing the impeller, also visually check if any other water pump parts show wear or damage. If the water pump housing has not been replaced in a while, it is sometimes better to just go ahead and replace the whole pump. The additional cost for the whole water pump assembly is minor compared to the cost/risk of engine failure due to severe overheating. That is my opinion anyways.
If replacing the impeller/water pump does not solve the problem, then you can test/replace the thermostats. The lower unit does not have to be removed to do that so there is no need to do that at the same time as the impeller/water pump replacement.
Installing a water pressure gauge is also a good way to troubleshoot overheating problems. Also, consider the possiblitiy that the alarm could be defective and the motor is not really overheating.
