Price the cruise indepedently of these coupons. They may ONLY be valid on certain fare codes (rates). IE, they may be getting that money out of you in the rates.
However, shipboard credits in that amount is not an uncommon incentive for the cruise lines to offer. So, there probably is very little fine print.
The coupons to watch out for are the ones that say they are only available under certain fare codes, rates, promos. I know of one cruise line that offers a $100 discount coupon everywhere you turn, but you can always get better rates under normal booking, as the coupon is only good off of brochure prices. Its just a trick.