They scan all suitcases on arrival at the ship.
If they find liquor they will not throw you off the ship- they confiscate it and return it to you on the last night of the cruise.
As you come back onto the ship from ports you have to go through the same type of scanners/patdown as you do at airports.
Not to say people don't come up with some ingenious ways to smuggle it on
but the policy is that way because they make their profits from services sold on board, not from your cruise fare.