Place corn in a sink full of cold water; weigh down with a heavy bowl; soak 30 minutes.
Meanwhile, heat gas grill to medium-high heat, or charcoal grill to medium-hot coals (mounding slightly on one side).
Transfer soaked corn to heated grill. Grill corn, turning frequently, 20 minutes. If using gas grill, move corn to warming tray. If using charcoal, transfer corn to a large platter and cover with a clean cloth to keep warm.
Tear off six 10-inch-square pieces of foil. Place one flounder fillet in center of one foil square. Season with 1/8 teaspoon of the Old Bay seasoning. Tuck ends under fillet, top with 1/4 teaspoon of the olive oil and one lemon slice. Seal foil tightly to form packet; repeat with remaining fish, Old Bay, oil and lemon.
Reduce heat to medium on a gas grill. Place fish packets on grill; on charcoal grill, group packets on side with less charcoal. Grill packets 18 to 20 minutes or until fish is white throughout. Serve with corn and butter alongside.