63 merc 650 copper water tube/overheating question

[Hairy_Vetch]

So far on this motor I have replaced:<br />prop shaft seal<br />water pump base seal<br />water pump and all gaskets.<br />Still have water in oil so I am in process of replacing shift shaft seal. I have recently treated this motor with seafoam. <br />When I pulled the lower unit this time I notice a large build up of black gunk at the top of my water tube where it meets with power head. I had to wiggle the water tube to get it out. I was getting water from the pee hole but not as much as I would suspect. What would cause this build up? I just want to make sure I am not overheating the engine.