You can't actually book the cruise while onboard. You must make your intentions known to the booking agent, then book within 3 days of getting off the ship. If it's over 11 months but less than 14 months, you must call within 3 days then call back at the 11 month window. I'm not sure the savings, maybe 10% or so of the points. I don't think DVC members get any shipboard credit that may be offered to cash bookings while on board..