In two separate saucepans bring water to a boil with 1 tablespoon salt and 2 tablespoons vinegar in each pan. (If you don't have two saucepans, you can execute this step in two stages using the same saucepan twice.) When the water comes to a boil, drop the shrimp into one pan and the scallops into the other. Cook the shrimp for 1 to 2 minutes, depending on their size, and the scallops for about 1 minute, until their color turns a flat white. Drain immediately.
Shell the shrimp leaving the tails on, remove the vein that runs down the center of the back, and put them in a bowl with the drained scallops.
Peel 4 oranges with a sharp paring knife, removing the peel and the pith. Slip the blade in between the sections to detach the pulp from the thin membrane that encloses each section. Lift out the membrane-less sections and remove any seeds. Set aside.
Detach the lettuce leaves from the head, discarding any bruised or wilted leaves. Wash the leaves in several changes of cold water and shake dry in a towel or in a salad spinner. Cut the leaves into strips about 1/2-inch wide. Cut the remaining orange in half and squeeze out the juice.
Combine the shrimp, scallops, and orange sections in a bowl. Add the orange juice, salt, olive oil, and liberal grindings of pepper and toss thoroughly.
Choose a shallow-- neither too deep nor too flat-- serving platter. Line it with strips of lettuce and top them with the shrimp, scallops, and orange sections, distributing evenly over them all the seasonings in the bowl. Serve at once.