Peeing does not mean its cooling on a OMC, also that engine has water valve on throttle. The heads at idle will run really hot to the touch on these models. Replace t-stat inside of poppet and replace gasket as these t-stat have a tendency to stick open and the gasket screws onto poppet...
i think that means that the system cant get to proper pressure so the water doesnt hang around long enough on the surfaces to transfer away heat, well ive read something like that many times on hereWouldn't sticking open allow it to cool better, or not get up to proper running temp?
Check the whole pump over, a new impeller might only be part of the issue. Also sounds like you dropped spark on one cylinder?