Marine: Boating, Sailing, PWC, GPS and Navigation - 4.3l engine overheating at idle.

I have a Regal medallion with a Mercruiser 4.3l engine and an alpha gen 1 outdrive. it was given to me by a co-worker. he told me that the engine over heats while idling, and that the Risers needed replacing. it doesn't make sense to me that the engine should overheat due to the risers. I got it started this weekend, and it does indeed get hot after idling (for quite a while I might add 15-20 mins) but give it some throttle, and it cools right down. does anyone have any ideas as to what the problem might be? could this be normal since the drive has a lower gear ratio, and thus not turning the waterpump in the outdrive fast enough to provide enough water to cool the engine at idle? I don't know the gear ratio of the outdrive. I do however, have another boat with a mercruiser 5.7l vortec v8, alpha gen 1 outdrive. no overheating issues at all. any ideas / help will be appreciated..

Thanks, Craig..

I'm not a boater, but my first thought would be to remove the water pump and make sure the impeller blades haven't corroded off.

The gear ratio of the transmission has nothing to do with the water pump, so that's not a likely cause.

The impeller in the fresh water pump could be compromised. Many techs recommend changing them every two years. They get damaged easily, even by running the engine for a little as 20 seconds with no water.

The impeller is nothing but rubber tines and friction.