The rest of the deck is storage, crew housing, and yes, it is usually the deck where you leave and re-enter the ship during port calls - though embarkation and disembarkation of the cruise itself (at beginning & end) will most likely be from a higher deck, usually the one(s) with more public areas. Personally, I'd see if you can opt for the next deck up where there are available cabins, unless you want to be first off the ship at each of the islands!