The water in the spark plug hole can cause the spark voltage to jump to the spark plug hole wall instead of going thru the spark plug. I think the water might cause boot damage if it stays in the spark plug hole for a long time...
This describes the spark arc'ing through the boot to the spark plug hole wall...
I think quite a few folks think the spark plug or the coil on plug goes bad so they replace both to fix the problem when only the boot on the COP is bad. I don't know if the COP boot can be bought as a seperate piece.
1999 Ford F150 Lariat Flareside Extended Cab, 4x4, 5.4L engine.