I think you are thinking about Future Cruise Deposits, FCDs. They cost $100 each. Booking, not sailing, must be made within one year of purchase. They are individual. If you are a couple then each one must have the FCD if each wants to receive the benefits. They are fully refundable if not used. When used the $100 is the full deposit (world cruises excepted) and you will receive some OBC according to the cabin type and length of the cruise, see below. Purchase of FCDs goes directly to your credit card, not to your on board accounts.
"Upon making your new booking, the appropriate shipboard credit will be applied.*
Future Cruise Length Per Person Shipboard Credit*
17 - 44 Days $125 Inside, $125 Outside, $150 Balcony, $150 Suite
11 - 16 Days $75 Inside, $75 Outside, $100 Balcony, $100 Suite
7 - 10 Days $25 Inside, $25 Outside, $50 Balcony, $50 Suite
3 - 6 Days $15 Inside, $15 Outside, $25 Balcony, $25 Suite"