Tammi - Looking at Personal Navigators for embarkation day on the three ships that have already sailed, it depends on which ship you are on. For the Dream, the Navigator says to call.
If you ever take a cruise on the older ships, and for the people who are planning to, the Navigators clearly say to show up at Wavebands / Rockin' Bar D from noon until 3:30, or call after 3:30. A few seconds of logical thought will lead to the conclusion that, if you give the rest of the passengers 3.5 hours to make their reservations before trying to get your own, your odds of getting a reservation are incredibly slim.