Thanks, all, for the many replies! Now we are considering doing our whale-watching tour via an NCL excursion (found out we had more excursion credit than we were aware of). If so, the size of the boat is out of my hands, so I think we'll just have to load up on seabands, Bonine, and green apples and hope for the best!
(If anyone has info on NCL's whale-watching excursions, I'd love to hear it!)