I have a 1958 Johnson. I had water cooling problems. I took off the thermostat cover and ran the motor. Out the small tube I got exhaust pressure and smoke. The repair manual states if there is an exhaust leak into the system it will not cool. You may even have good compression readings and it will not pump. After much frustration we took the power head off checked all passages and nothing. Turned the driveshaft in a bucket of water with a drill and got great flow. Upon taking the head off found 3 head bolts only finger tight. Replaced the head gasket torqued all the bolts down and water pumps great. ANY LEAK around the head gasket and it will not pump water. Im happy now but it was frustrating. It takes a long time running for these motors to pump water when you first start it up. That is my experience, hope it helps.
