I just came back (to possible 12 inches of snow tomorrow) from a 10 day Caribbean cruise (our first cruise). I thought I would provide my opinions and observations regarding photography and going on a cruise.
First question that many ask is should they even take their DSLR. IMO the answer is yes. You bought the camera as a tool to take fantastic photos, why would you not take it? I have a lowpro back back that carried my 7d, 17-50 lens and my 70-200 lens. I just brought it on the plane as a cary on - no issues at all. I also used the lowpro to keep bottled water when touring the islands.
The next common question is what lens should I bring? As I mentioned before I brought the 17-50 and 70-200. I soon discovered the limitations that the 70-200 would bring, so 99% of my photos were taken with the 17-50 lens. There were even times that I could have used something a bit wider, but the 17mm worked fine for me. I may leave the 70-200 home on my next trip. There are a lot of low light situations on the ship so I highly recommend a fast lens (I did not bring my 430ex).
Another item that I used often was a polarizer. One thing that I recommend is that you find the deepest colors and back it off a bit. If you don't it can actually make the photo look over saturated in my opinion.
I did not bring a tripod but another photographer on our ship had one. It came in handy for him one night when we got off the ship and took long exposures of the boat. Other than that I did not see any use for one.
Plenty of cards. I took 4 CF cards with me. I ended up taking more video that I had planned and it really ate through my cards. In the end I had enough, but I had to keep an eye on it. One thing that I did at each new island/port is to take a photo of the daily cruise news letter with the name of the island on it. That would provide me an idea of what the days photos originated from (I took over 800 photos).
Just my observations - hope it may help someone who is going on their first cruise!