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.
The spark plug gap for a 2003 Mercury 125hp outboard motor is .040"
A 50hp Mercury outboard motor typically has two spark plugs. This configuration is common for two-stroke engines, which often use one spark plug per cylinder. If you have a specific model or year in mind, it's always best to check the owner's manual for precise details.
spark plug gap for a 1996 mercury 30 hp outboard motor?
What is the spark plug gap for a 9.9hp mercury outboard motor
100 hp searay
3 spark plugs
.025 Champian J4J (cold) to J8J (plugs) Points .018 - .02
.040
NGK bpr7hs-10
The spark plugs on a 1988 model, 70hp Johnson outboard, should be gapped to .030in.
Ngk bpz8h-n-10
The recommended spark plug for the 1977 model, 7.5 hp Mercury outboard is a Champion L78V, or NGK BUHW-2. Both brands are Perma Gap type plugs, with the Champion being the manufacturer supplied plug back in 1977.