Yes it is always parked on level ground. Its becoming a real pain. I don't understand why its just the rear cylinder on drivers side only. I might try and cut a shield out of some tin to divert the water.
Try that spray silicone, it works pretty good. Reason i ask about level ground, had a customer that parked with the front end down hill, bad problem with wet coil/plug boots when raining.. Turned vehicle around, solved problem! Was hoping you could the same.
This happens a lot with 4.6l-5.4l engines. Driving through water puddles and such the water above the hood on the outside of the car as you can see. Well the water also goes in the engine compartment, you don't see. I have put some sealer on the top ring of the spark plug boots, where it contacts the top of the spark plug hole. This seems to help with the water problem.