could be a bad water tube gromet or a bad powrhead gasket corroded exhaust plate. in any case the powerhead will have to be removed to fix any of these. before you go there try to tighten the 8 powerhead bolts under the outter plate (Remove the 4 Screws)and see if it stops leaking after tightening them
check all gaskets and plates. even the next level replace the gasket there and check the water tube gormets for wear. Being you have the powerhead removed it would be in your best interest to change gaskets on the exhuast cover on the side of the engine and if possible the waterjacket or head gasket. You cannot get to these areas on some engines with the powerhead installed. It would be a shame if one of these gaskets lets go after you came this far. also can you post a picture of the powerhead base? also check to see if all surfaces are flat with no corroded sections
