Each cruise line is a little bit different on what type of future cruise amenities they'll offer while onboard your current cruise. But my advice is that if you plan to do another cruise with that company, then always take advantage of the future cruise program as you'll get the chance to get a reduced deposit along with onboard credit - both of which are always a good thing.
You do have to pay for the deposit when booking - they won't charge it to your onboard account.
And if you have a travel agent, let them know when booking so your agent can service the reservation for you.
Travel Agent/Cruise Specialist w/12 yrs exp and 47 Cruises on 11 cruise lines! Favorites: Paul Gauguin - Tahiti: Uniworld River Cruises - Europe; Celebrity Solstice-class ships; Holland America - 12-nights Baltics & Russia; RCCL - 14-nights Greek Isles, Turkey, & Croatia; Holland America - 14-day Alaskan cruisetour; 10-night Canada/New England cruise; 21 days Hawaii w/7-night NCL cruise; Oceania - 25 days in Asia; more than 3 months touring Europe by train. And many all-inclusive resorts!