You won't be able to carry any food off or onto the ship. I always eat on the ship unless I'm on an all day excursion. There's all a place a person can food and drink. because I know it good food. When you mention the Aquatic World excursion are your referring to Coral World? If so, you may want to consider going to Coki Beach instead of Magens Bay because Coke Beach is right next to Coral World. I would suggest taking a taxi there - you'll probably save some money rather than booking a tour. Check out Coral World hours and prices
HERE
Sonny