Why does water leak from the spark plugs on Mercury 80 hp outboard motor?

That sounds rather serious, and I'm assuming you can only see the water if you pull the plug. If that's the case it'd indicate that you have a ruptured head gasket that's allowing coolant water into the cylinder, which should cause the engine to run rather poorly. If you can see the water w/o pulling the plug it could still indicate a bad head gasket or perhaps a crack in the head as well.