I can't see how water sitting around the base of the plug would cause a
misfire - there's plenty of surface of the threads in contact with the head and
it can't be completely corroded between them.  I think it's eyewash, but anyway,
undo them and clean the threads up with a wire brush, put a touch of oil on them
and screw them in again. While they're out, look at the colour of the bit that's
inside the engine - should be a nice greyish/off-white/straw colour (typical
range of colours) that shows things are OK.  At the very least, you may see one
that's blacker than the others - that's the one that is misfiring - check the
gap is similar to the others.  Maybe the plug itself is faulty - rare but it
does happen.  If the ends are eroded, change them.

BTW, you might like to sign up to a Ford newsgroup - they may know more about
problems with your Ford than a VW group!

Is your engine losing water?  You need to find where it's coming from.  If the
corrosion is crusty white or bluish, it's coolant.  If just rusty, it may be
rainwater or been splashing up from under the car.