Caddyman said
“This engine design places the spark-plugs down inside deep wells in the cylinder heads. The spark plug boots attempt to seal the openings.”
The Grand Marquis uses the SOHC 4.6 V-8. It DOES NOT have spark plug wells in the cam covers.
Water in the spark plugs makes no sense. Are you sure that’s how they phrased it? If so, you need a second opinion from a reputable shop. Don’t share what the other shop said, just let them diagnose it for you. Look here Mechanics Files | Car Talk for a shop near you.