I would never book this far in advance. Just too many unknowns.
You may decide you don't like cruising anymore, you may not be able to get medical coverage for pre-existing conditions, the cruise line service may go significantly downhill, the cruise company could go bankrupt, the itinerary could radically change, you find that there is a family commitment come up for the date of travel, etc.
Also, the non-refundable deposits seem to be $500 plus so a lot of money to lose if you decide not to go.