engine overheat

I was cruising along at 4000 RPM when I glanced down at my guages. To my surprise my port engine was reading over 200 degrees F. I did not hear any alarm and was surprised the engine guardian system didn't shut the engine down. I shut it down immediately and was able to limp back to my marina on the starboard engine. I checked the engine with a heat gun after it was off for 20 minutes and I got a reading of 187 degrees. I just had new raw water impellers put on this season along with new thermostats. . This is the 5th season for this boat in salt water. Could the risers clog up and create this problem so suddenly? The engine temperatures were fine last night when I went out for a ride. Anything else I should be thinking about? How long does it take running a mercruiser at 200 degrees before serious damage occurs? I doubt it was more than a few minutes before I noticed the temperature.
I have 4.3 L MPI MerCruisers

Re: engine overheat

200 degrees isn't overheating. Remember that's a car engine in there and it typically runs 195-210 degrees.

Regardless, your cooling system definitely isn't working properly.