It is very rare for
Disney Cruise rates to go down. Special fares such as Florida Resident or Military may be introduced, as may restricted fares requiring non-refundable payments at time of booking, but generally, booking as soon as cruises are announced gets you the lowest non-restricted fares.
Your best option is to book the December cruise you want now, and purchase a Placeholder reservation on your cruise in March. Then, when you get home, check whether the current fare minus your onboard booking discount will be less than you paid for the December cruise. If so, you can cancel the earlier reservation and convert the placeholder to a new reservation. If not, you can keep the earlier reservation and apply the placeholder to another cruise (or let it cancel if you don’t take another cruise within 2 years).