There is one on the intake manifold right at the front, it acts as a vent, unscrew it before you open the drain handle or right after. There is also a blue plug on the water distribution housing, the thing on lower front port corner of the engine with the orange drain hose on it that the water comes out of when you open the blue handle.. Instructions say to pull that plug if you suspect that there may be residual water in that housing, that is odd to me the Merc leaves it to the individual's discretion.
As Walt and others have mentioned, the engine block may not drain well after an engine has many seasons on it with just those drain points being exercised. That is why the added precaution of removing the hose connections at the engine block drain and prodding the openings with a long pick or skinny screwdriver is being recommended.
I hate that system, it has more water in all the plumbing than it does in the engine block it seems. A LOT of water comes out when you open the handle, so you tend to think it is good to go, especially when you are used to engine systems that just have the drain plugs. Hard to believe Merc did that to "us".
I had to replace that housing on mine this year at beginning of the season. I R&R'd the engine last year when the coupler failed in that new-to-me boat in my sig pic after 6 trips. I know I bumped that housing some along the way, I did not have a lot of room, maybe stressed it somehow. Or it may have just frozen as i did not take that plug out, we had 3-4 days of 16-18 degrees last winter here in North Texas. After my first shakedown trip this season after about an hour on the lake, I had about 3-4 gallons of cold water that came out of the bilge when I drained it. Ran it on the hose at home, reached under that housing and felt the water trickling out. I removed it, I could not see where the crack was. Put a new one in and all dry again.
