I was at my boat today and looked at my 3GM30F and the service manual that's on the boat also. That plate gives access to the cooling passages in the head. In the raw water cooled engines, there is a zinc installed in a similar plate (with threaded hole for the zinc). In our engines, it's just a flat plate with a gasket. Sounds like yours is leaking a little. Doesn't sound like a big deal. Your can either ignore it until (or if) it gets worse, or change the gasket now.