First, let me say the Mariner is an absolutely fantastic ship and you are going to have a great time. Category D2 cabins are a comfortable 202 sq. feet and the closet/storage space is more than sufficient. There is both full length and half closets for hanging items and a number of deep shelves for storage. There should be plenty of hangers in the closet - but no need to bring your own, if you need more - just ask your cabin steward and they will be delivered promptly. You might want to read Tom's review of the Mariner on our parent site CruiseReviews.com. With lots of details and pictures you should get a real feel for the ship. Here is the link:
As for Port Canaveral. Cruise Connection is a bus that offers transportation to and from Jacksonville. If you decide to drive there is parking at the port for $10 a day. Another option is to drive down a day early and stay at one of the reasonably priced hotels overnight that offer free parking during the cruise.
Hope this helps -