I tethered when I had a Treo phone thru AT&T - if I was in home airspace, there wasn't any roaming fees . . . which included St Thomas, San Juan and any US turnaround port. Check and see if your provider has an unlimited data plan and coverage in the areas you'll be in. And yes, you do have to be careful or you'll get nailed with the roaming if you're not watching the signal.
I have an iPhone now and can quickly check to see if there's any accessable wifi in the area, and then open the laptop and go (or just use the iPhone).
As soon as you get on the ship, you'll have tons of help and advice from your fellow crewmembers . . trust me on that one
You'll be working with a diverse international group who use services like Skype to keep in touch with their friends and family back home. They'll know where to find the best free or cheap places.
Enjoy Alaska - I did two summers there . . it's a stunning run!