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.
47 Cruises & Counting! Favorites: Paul Gauguin to Tahiti: Uniworld River Cruises in Europe; any of the Celebrity Solstice-class ships; Holland America for 12-nights in the Baltics & Russia; RCCL for 14-night Greek Isles, Turkey, & Croatia; Holland America for 14-day Alaska cruisetour; 10-night Canada/New England cruise; 21 days in Hawaii including a 7-night NCL cruise; Oceania for 25 days in Asia; & 3 months touring Europe by train. And many days spent in all-inclusive resorts!