I suggest you check your water pump impeller. Replace the whole pump
housing when you replace the impeller. Just a few bucks more and you
don't have to worry about that part for two more impeller changes. The
impellers need changing every 2 years because they hold a shape that
lets water get past after being in the housing for so long. They are
hard rubber., I had the same problem with mine. When I was moving at a
decent speed my temp stayed low but when I slowed down like in a no wake
zone it heated up. I had to put it in neutral and rev it up until it
cooled off. If I started it and let it run for any time without moving
it heated also. I hope this helps.

Bobo wrote:
I recently bought a 1990 Searay 180 Bowrider I/O V-6 (4.3 L) Mercrusier. I
have noticed that the baot always runs cool (140-145) when I am at a decent
speed. When I slow down to 3-5 mph, or when I start up after being shut down
for a few minutes, the temp goes to about 220. Once I get up to speed, it
goes back down to 145 again. Also after I have shut down, on trying to start
the boat I get the alarm and the temp is pegged. My mechanic says not to
worry. Any Ideas?