I am not aware of any specific problems with your exhaust but, would like to share a thought. You say you saw what appeared to be condensation in the pipe. If you operate in cold water that could be the problem. Mercury encountered a condensation issue with the 420's and GIL's. They installed a diffuser ring between the tail piece and the manifold to catch the drops so they would burn off and not fall in the manifold and lead to valve issues. The HP 500's came with them as well.
#24 is the ring
http://www.mercruiserparts.com/Show_...NIFOLD%2FELBOW
If you can not find any other explanation maybe this is something to look in to.