2004 Mercury 150hp XR6 temp alarm!!

[BGUN71]

First, Hi everyone, I've used this site in the past for reference and finally signed up. Thanks for all your help in the past. My problem started yesterday, engine idled for a about 10 minutes good water pressure, good stream from tell tale, ran for approx. 1/2 mile at cruising speed approx. 2500 - 3000 rpm and got high temp alarm (constant buzz). I shut it down for no more that a minute or two, turned the key back on, no alarm, so I restarted and putted around at idle for about 5 min, still good water pressure and stream, got back on top same cruising speed and distance, alarmed again, (repeated the same process again on the way back to the launch). Any ideas on where to start trouble shooting?? Again, is a 2004 Mercury 150hp XR6...