First, I would not suggest those sea bands - they work based on accupressure and don't have the success rates of other methods. One of the best things to do is take ginger pills - no side affects like the pills or patches.
But no matter what you decide on, you need to start about 3 days before the cruise. If you want until you're on the cruise or until you start getting motion sickness, it's too late.
Depending on your itinerary, most of the roundtrip cruises stay within a relatively protected area, so chances are you won't encounter rough seas. Not to say you can't, but doesn't happen too often. If you're doing a one-way, chances are a little better, but still not all that often.
However, most of the ships cruising in Alaska are relatively new and have stabilizing devices should rough seas be encountered.
And lastly, if you're prone to motion sickness, then talk to your agent about getting a cabin more in the middle of the ship where you feel less motion than in the front or back of the ship.
__________________ Travel Agent/Cruise Specialist w/13 yrs exp and 48 Cruises on 11 cruise lines! Favorites: Paul Gauguin - Tahiti: Uniworld River Cruises - Europe; Celebrity Solstice-class ships; Holland America - 12-nights Baltics & Russia; RCCL - 14-nights Greek Isles, Turkey, & Croatia; Holland America - 14-day Alaskan cruisetour; 10-night Canada/New England cruise; 21 days Hawaii w/7-night NCL cruise; Oceania - 25 days in Asia; more than 3 months touring Europe by train. And many all-inclusive resorts!