I took an Alaska cruise not that many years ago, but was still shooting 35mm transparency film. At the time I had the flagship Canon's and mainly shot Fuji Velvia (iso 50) and Fuji Provia (iso 100, though I did push process some of the film to 200 and 400). I had three Canon zooms, covering from 17mm to 200mm (all f/2.8) and my 300mm f/2.8 with both converters (1.4 and 2.0). I used the 300mm lens extensively on deck, while the ship was in motion, mounted on a heavy Gitzo carbon fiber tripod. I also used the tripod for most shots with the other lens, in particular the 70-200mm. I don't think I lost a single shot to vibrations or movement of the ship, and believe me, I am extremely particular when it comes to sharpness. I was careful to shoot from portions of the deck that were less prone to vibrations, to stay out of the wind and spray, and it worked out fine. Yes, the above package was not light weight, but the photographs were my primary reason for the trip. And, I continue to license photos from that journey to this day. Digital cameras that allow us to shoot at higher ISO's with better image quality will make it that much easier to shoot from aboard ship. The reach and speed of the 300mm lens, along with the converters, was very helpful. But, lens length would be somewhat dependent on your shooting style. Make sure that if you buy new equipment you plan to use on the trip that you purchase it well in advance, and familiarize yourself with it thoroughly before departure. I did a month long land shoot in Alaska this summer and took all of the above gear, except 1Ds Mark III's rather than film bodies, a Canon 500 f/4 (loaned from Canon CPS), Canon 24mm TSE (for landscapes), Canon 100 f/2.8 macro (close-ups) and a Leica M9-P with four lenses, and two heavy Gitzo carbon fiber tripods (one with a Wimberly head for use with the long lenses). It was a lot of gear, most of which I shipped ahead to Anchorage, but once it was organized in my rental truck/camper I was glad to have it all and made use of every piece of gear. I don't think there is one set answer to your question, you have to be comfortable with the gear you take, and willing to drag it along. If it is sitting at home, or in the ship's cabin, you will never get that special shot. And, how many times do you plan to take an Alaska cruise? Bon voyage and happy shooting!