Combine water, vinegar, salt, mustard, pepper, and bay leaves in a large Dutch oven; bring to a boil. Add lobster tails; return to a boil, and cook 5 minutes. Remove lobster tails from water with kitchen tongs. Rinse under cold water. Drain.
With a sharp knife or kitchen shears, cut down outer edges of tails to remove membranous covering and expose meat. Remove the entire tail section of meat, and cut into 1/2-inch-thick slices. Return meat to shell; cover and chill.
Place lobster tails on individual serving dishes; top with Cucumber Sauce. Garnish with cucumber slices, if desired.