At least try to order it at the time you place your regular order so the waiter doesn't have to go back to the kitchen for it.
On a recent Princess cruise , everyone at our table for ten ordered crab legs. We then asked the waiter for seconds and he brought a a heaping plate of crabs legs (must have been 3 pounds) and we all dug in family style.
I have never heard a waiter say no to a request made at a dinner table.