I recently bought two outboard motors at very good prices because the owner thought they had significant cooling problems. The only problem they both had was an insect in the South called a dirt dabber. They like to fill the water exhaust with mud, insects and so forth. The water no longer comes out and you think you have a problem. Merely take a small piece of wire of appropriate diameter and gently remove the debris. Works most of the time ! I did have a Mercury that refused to let water flow out until about one and one-half minutes passed. Seems it had a thermostat that held water in until temp rose. Good fishing and skiing !