Yes, there's an area called "NextCruise" that you can visit and book while onboard to book a future cruise. Honestly, the discount isn't any different than what you see online most of the time (50% off 2nd guest, etc.). The main incentive, which isn't really much of an incentive to me, is that the initial deposit amount is reduced. There may be a little extra onboard credit, but it isn't significant.