How long are you going, how much do you drink and do you plan to buy anything on the ship (booze to take home, tobacco products, clothing, souvenirs)? There are lots of places to spend money on the ship but you must use your sail and sign card. Have you given any thought about gambling?
You should be able to charge just about everything on board to your S & S Card. The only cash you might need on board would be for tips (e.g. for room service if you request something special). On excurions the drivers/guides expect to be tipped also.