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?

Yea, GET A NEW MECHANIC AND FAST