You can get cash with your seapass in the Casino. I'm not sure if there is a limit to how much you can get but I have gotten "tip" money there when the line at Guest Relations is too long. You also can add your tips on the seapass card at the Pursers desk.
There is a cash machine in the promenade but it charges a $5 fee. Best bet is to use the casino. Just put the card on the table and tell them how much you would like.