If you book a cruise while on a cruise you get an onboard credit for that new booking. It varies in value depending on cruise length. $50 for 3/4 nights, $100 for 7 nights, etc. I think it tops out at $200 or $300.
I use a cruise specialist for my cruise bookings. If I happened to make a booking on a cruise, I can have the booking assigned to my agent and she gets the commission.