The only branded ones I'm aware of are
Royal Caribbean's Visa and
NCL's Mastercard (both offered by MBNA).
RCCL's works like this: 1 point per $1 spent. Rewards start at 5K (upgrades). The main rewards are as follows: $25K in spend gets you 1/2 off a cabin on a 3/4 night cruise, $50K in spend gets you a free cabin on a 3/4 night. $75K in spend gets you a 1/2 off for a 7 night, $125K gets you a free 7 night for 2.
Depending on when you go (if you go at peak times), it can be a great deal, as an outside cabin on a bahamas cruise can run over $900. If you go off peak, it is better to pay cash for the cabin (as they can get as low at $500).
NCL is better, giving you 3 points per $100 in spend. They offer straight dollars off, starting at $500 off for 500 points, it goes up to $3000 off for 3K points (or $100K in spend).
I'm more inclined to get a generic cashback card that gives you the 1% in the form of cash back (which does not lock you into a particular cruise line).
But, if you're an NCL fan, that card (at 3% back) is a very, very good deal.