Do you have a TA? Give him/her your preferences before you leave so you can book at opening day prices then adjust rooms, etc as needed. Talk to your TA to get some ideas of what itineraries may be available. They usually have some inkling.
If you do this, you lose any onboard booking incentives. Best to take a little time out of vacation and go to the onboard booking desk. As Trera said, they do open the bookings to Gold/Platinum the same day they would be available on land. We were onboard when they announced the fall 2016 itineraries (although I didn't think they were great about communicating what was going on to everyone onboard) and we waited until afternoon the day bookings opened and were able to get ours booked on the Gold/Platinum day, with very little wait and get our onboard incentives.