Bring 5 quarts water to a boil in an 8-quart stockpot; plunge lobsters headfirst, 1 at a time, into water. Return to a boil. Cover, reduce heat, and simmer 2 minutes; drain well.
Place potatoes in a saucepan; cover with cold water. Bring to a boil. Reduce heat, and simmer 10 minutes or until almost tender; drain.
Place potatoes in a large roasting pan; add remaining 1 cup water, lemon juice, mussels, clams, parsley, kielbasa, thyme, corn, and onions to pan; arrange lobsters over potato mixture.
Sprinkle evenly with seasoning; cover with foil.
Place pan on grill rack; cook 18 minutes or until clams and mussels open. Discard any unopened shells.
