Almost surely you have a bad impeller. Even though I have long known not to
run it dry (a friend did that to a 188 Mercruiser in the 1970's), I was having
the same symtpoms: Overheat at low speed, cools down at speeds over about 1500
RPM. Shop replaced the impeller, and no problems since.

I suggest that you contact the local Power Squadron . . . not meaning to be
snide about it, but the water is full of people who "think" they know
everything . . . and don't.

I've been affiliated with the Power Squadron her for 29 years . . . and I learn
new stuff all the time. Don't be afraid to learn!