Re: 1984 115 mercury l6 outboard
Probably just a leaky gasket at the cylinder block water jacket cover. This is not a cylinder head, just a cover over the cylinders and water passages.
An emergency fix if you have to run the motor, is to clean the area with solvent and fill with automotive-grade "RTV" silicone rubber. Of course you wouldn't block the area where the spark plug seats on the block, another way would be just to leave the spark plug installed and RTV it in, effectively sealing the leak until the next time you remove the spark plug.
Do not use silicone containing copper, as copper against aluminum can promote corrosion.
The correct way, of course, to fix the problem is to remove the cover, scrape off all gasket material on the block & cover, degrease and reinstall with a new gasket.
It is possible to do this repair without pulling the powerhead, although the bottom bolts can be a bit difficult to get at. If you remove the little "horse collar" trim piece below the lower cowling (held on by 4 screws at the bottom), and remove the rear cowling bracket, it'll let the lower cowling drop down a bit which will provide better access to the lower bolts.
An Extreme Caveat on this, especially for the "Uninitiated", is that the 1/4-20 bolts are very easy to break if they are the least bit stuck or seized in place from salt or other deposits.
Any bolt that doesn't break right loose with a bit of pressure from the wrench, must be heated with a torch to free it up. Elsewise you'll be drilling and tapping bolts out of an aluminum block and that ain't fun! A handheld propane or mapp torch will do the job nicely.
The P/N of the cylinder block cover gasket is "98185 3", "98185" being the "root" part number with "3" being the latest iteration. Any gasket in the supercession thread back to 27-98185 will work, but the most recent part number is the upgraded part.
When you install the gasket, if it's got a coating of gasket glue on it, install dry. It'll look shiny.
If it looks flat like regular old, plain, gasket paper, you can still install dry but I prefer to coat both sides with Permatex No. 3 Aviation-Type gasket dressing.
If the flat sealing surface of the block, where the spark plugs screw into, are a bit corroded/chewed up, you can promote sealing by using a very thin coating of aforementioned "RTV" silcone sealer on that area only
Just be sure to use a sparing amount, realizing that any excess that's squeezed out may eventually break loose and circulate around in the cooling system, and big chunks will likely plug up something, somewhere. A thin layer won't do that.
HTH & let us know how it goes...........ed
