Overheating, flow problem is past the thermostat

[cassenbl]

2003 mercury 40hp 4 stroke 3 cylinder serial #0T783233. Engine overheats at idle after a couple of minutes. Replaced water pump twice. Ran scope up water tube, everything OK. Engine is overheating, too hot to put hand on. Replaced thermostat. Will overheat with thermostat in or out. Have a lot of flow coming out of telltale. Yesterday, I wanted to check water flow so I removed thermostat cover, took out thermostat and left thermostat cover off. Started engine and water started come out of thermostat hole and telltale. Engine was nice and cool. I ran it 20 minutes and no alams and everything nice and cold to touch. Put thermostat cover and engine overheats again. My guess there is some kind of obstruction so water can not get past thermostat. I have not taken the exhaust cover yet. What is past the Thermostat? I do not think this model has a poppett. Service manual does not show one but does not that system well. Any ideas?

[boats.net]

Remove exhaust cover and look for restrictions. Could be blockage in water passages, look for corrosion between exhaust chambers and water chambers.