They usually dont check (ive never been checked) when leaving the ship in port..Only when going back on ship they check. My sister and I always took snacks from the buffet and never been stopped. Cerial boxes etc and some fruit, yogurt. Not saying its right to do. But ive done it a few times with no issues. When ashore if I need other drinks, I just buy a canned soda or somthing in a can or bottle.