In a large roasting pan (about 12"x16"), place a one-inch layer of seaweed. Parboil onions and potatoes. Shuck corn for innermost husks. Place Maine lobsters on the seaweed, arrange corn and onions between the lobsters and the sides of the pan. Place clams, mussels and potatoes gently over the lobsters, keeping the top of the bake level. Cover with remaining seaweed and add water to the pan. Cover tightly with foil and place on the stove or a preheated grill to cook (if using a grill, make sure the coals are very hot). Start timing the bake when you first see steam. Cook covered for 15 minutes. Remove from heat but leave covered for 5 more minutes. Serve with lemon, melted butter or margarine or your favorite lobster accompaniment.
