Same thing happened to me this spring on the missouri in march. One would think with 33 degree water temp it wouldn't over heat but it did. Replace the water pump impeller and there hasn't been an issue since. I believe the impeller was $25 on overtons, and it was an easy install. Just look up a youtube video on how to do it, pretty self explanitory. It took me about an hour to do it, with most of that time being spent on trying to get the lower unit back in because I didn't realize how important it was to put it in gear before you start. When the lower unit is in gear you can just spin the prop to get the splines to line up again.
Read something about on the Optimax's the impeller should be replaced every few years.