All the previous suggestions sound right on. Last year I was having hot engine issues, did everything suggested, plus flushed the fresh water cooling system, replaced thermostat, impeller, & several hoses. Then ran a product called Barnacle Buster Concentrate from Trac Ecological through the system. I let it sit overnight and flushed it out, no more worries. I plan to do this probably every other year now.
TRAC Ecological Marine Products