Since you mention DVC and onboard booking, I will add this...
DVC Members cannot book another cruise with DVC points while onboard. Because of this restriction, a few years ago DVC added the following option.
You can call DVC Member Services with 7 days of disembarking your cruise and book another cruise using points and receive a small discount on the number of points required. There is no exact way to know what discount you might receive, it likely varies by stateroom category and sail date. Based on reports from the boards, the discount is nowhere near 10% and is usually so small (a point or two, maybe up to five) that it really doesn't make much of a difference. The other "Book onboard" benefits such as onboard credit do not apply.