At the end of the day you made a mistake by inappropriately preparing your documentation for the cruise. What you may not be aware of is that the cruise line is required to ensure your documentation would meet the standards set fourth by Homeland Security. When cruises return to port in the US (at the conclusion of a cruise); you actually have to physically go through US Customs at the pier; so if the cruise line were to allow you to board with inappropriate documentation; then you could have been in bigger trouble when attempting to re-enter the country when the ship returns to port. Keep in mind that NONE of Royal Caribbean vessels are registered in the USA; as such, as soon as you step foot on their ships; it's like stepping foot in the country by which the ship is registered.
In my opinion, Royal Caribbean has zero responsiblity to give you anything in return for your error; however, I believe their jesture of a 75% coupon was more than fair.
This would be no different if you were attempting to fly to the bahamas on USAir and when you got to the airport the airport agent denied you boarding to the plane due to inappropriate documentation.
Hope that's not too harsh; however, that's just the way the facts fall.