With all the debris in the Chesapeake since Irene and Lee, you might have sucked something into your cooling water intake.
This happened to one of my dockmates recently.
It doesn't take much of an obstruction to cause overheating, especially if you have freshwater cooling. You might not be able to eyeball the differance.
The places to check are the exterior strainer/grate, the inline interior strainer, and the inside of the end caps on the heat exchanger. A different friend had a marginal overheating problem that he fought with for a year until he discovered a fragment of a paper towel stuck in one end of the heat exchanger!