Never had to deal with this before, so I didn't know what to expect. There really wasn't any issue here at all (we did have some minor issues at the airport). The cabin steward provided customs forms the night before, and we didn't have anything over the limit to declare. We didn't take or buy alcohol, so that wasn't an issue.
I would recommend a travel pouch for travel days. My "ID and Boarding Pass Holder" came in tremendously useful. There is a slot for driver's license, passport, and boarding pass; there is a zipper pouch on the back for cash, credit cards, etc. Rope included to wear around your neck. It kept everything pertinent close at hand for the hustle & bustle of boarding planes and ship.
Atlantis - well, know going in it's a large resort, designed to keep you there all day if possible. The excursion was scheduled for 3 hours, but it took nearly 4.5 hours. Just getting through the lines at the end took over an hour. The problem was photos; everybody wants pictures of the dolphin interaction. Every group's photos are together in a batch, and you must look through the entire batch to find the photos of you/your group (they are not always together). Each batch contains photos for at least 18 guests, and many shots of the dolphins doing jumps, etc; there are so MANY photos to look through. That whole experience really cheapens the feel of the excursion. Oh, the photos are pricey, too: 4 pictures = $60 (the cheapest package available). Warning: the lines are long & dull. With kids who are already tired, they have a hard time sitting still; the parents get grouchy. Patience is greatly required.