That's not a cyl head at the back of the block, but merely a cover. At the top of each cylinder is a flat machined spot that the cover seals against, where the spark plug screws in.
The cover gasket is leaking and water's spilling onto the spark plug.
Remove the 1/4-20 bolts holding the cover in place and install a new cover gasket. Pick up a gasket scraper at your local auto parts store or a good razor-blade-type paint scraper from Wal-Mart (cheap and what I use).
Mercury part Number 27-98185-3 or Sierra Marine # 18-2807 (think NAPA).
Here's a link to a picture of the gasket:
http://www.mercruiserparts.com/images%5Cmer%5C830.jpg
Note, if you've run this motor in salt water or alkali lakes etc, you may have a lot of deposits around the bolts. If they require more than normal hand-tool force to extract, STOP and get a propane or MAPP gas torch from Lowes or Home Depot, etc, then heat up the area around the bolt to free up. You don't wanna break any bolts! Then you'll be drilling them out and installing Heli-Coils!!!
You'll need to pull some of the cowling supporting structure off the motor to gain access. The same caveat applies as to heating bolts to get them out.
If you do have stuck bolts, smear on the threads a thing coating of waterproof boat trailer bearing grease and they'll never get stuck again.
It's actually a very easy job if all the bolts come out without a fight. Gasket is not very expensive, either, under $10.
HTH & Happy Wrenching.........ed
mariner water leak
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