Hey! I used to work on a cruise ship as a Photographer from jan 06 to may 07 and I can tell you that it totally can be done! I saw MANY passengers with full on 'pro' equipment and nobody even batted an eye. I think they mean that no studio gear and lighting is allowed on, but in this day and age, EVERYONE has a camera! You are bound to have the occasional Amateur with a Pro Camera and lenses! Nothing can stop them!
Think of it this way, as a Wedding Photographer, even I get a vacation every now and then... do you think I would leave my camera at home? Heck no! Let them TRY and stop me! Its in my blood and I would want to take some nice photos of my trip!
If for some reason they wouldnt let your photographer take photos walking around the ship (which I could understand), NOTHING can stop them from taking photos on land. Its out of their ... 'jurisdiction'?
I have been to Grand Cayman many times, and if that is where you are getting married, you will have an absolute ball and there are TONS of great photo ops around the islands!
I would keep it as big of a secret as you could though... no need telling everyone who your photographer is
if you have any more questions please let me know, I'd love to help out!