Larry in Miami is 100% right. If you get there by noon, they'll be happy to let you on. Remember that your boarding day is the first day of your cruise...so get there early!! Most things are clolsed but you can do the pool and you can certainly walk around and tour the ship. Remember the kast day you're going to eat and then they will throw you off!!
Many lines have EARLY DISEMBARKATION..if you can caryy off your own bags, then they'll let you leave as soon as the ship clears customs. Otherwise, it "hurry up and wait!!"
Have a GREAT cruise!