[narual]

I took an 18-200 and a 35mm 1.8 on my honeymoon (was still using Nikon then), the second week of which was a caribbean cruise.

I tripped the first day while hiking in the rain forest in puerto rico and broke the 18-200, so I shot the entire honeymoon with the 35mm.

There were a few times I wished I had a wider lens. There were a few times I wished I had a longer lens. (sometimes just because I tend to use them to read stuff at a distance, not even to really photograph it) The only time I really felt screwed by not having one was when I was photographing surfers in Rincon - I ended up using my wife's old point & shoot that had a 10x zoom on it, and it barely did the job.

Don't cart 800 lenses with you. Take something reasonably wide and fast. Your 16-50 is perfect. There's not a lot of use for the telephoto lengths on the cruise ship itself, except if you like standing on a high deck taking shots of girls in swimsuits, or photos of the uninteresting areas where all the tourists are leaving the cruise ship to go out adventuring. For excursions, take a superzoom of some kind. The 18-135 is good since you don't have to worry if it rains, which happens a lot. If you take the 50-135 or 60-250 instead, maybe toss a fast prime in your pocket, possibly in a ziplock bag in case you get soaked.

So far as waterproof goes -- if you're just snorkeling, you could try what I did -- get a GoPro Hero and mount it on your body somewhere, and set it to record video and simultaneously snap a photo every half second or so. I did a chest mount. If you're actually diving, you'll probably want something with a flash. My local dive store rented out the top end of the SeaLife cameras, with the strobes and so on, for ... I forget how much, but it was really, really surprisingly cheap. Like, I think it was $60 for a bit over 2 weeks. If only their cameras shot in RAW. It's worth looking at -- they even offered to show me how to use it properly for free.