Every time you get off and on the cruise ship at ports, your cruise card MUST be scanned. Have it out and ready with bar code side up when you get in line to get off. Do not wait till you are at security to dig through your bag looking for it and slowing up the entire process!
To get back on, you will have to show it again probably twice. Once face side up with the ship name showing to get through the port security (A photo id is also usually required and rarely a passport - they will tell you which ID you need to get back on the ship) And then again, bar code side up going back through security on the ship.