I always get the shot. It does not hurt -- a jab in the rear-- but it does put you to sleep for several hours -- actually a very nice sleep! I have always waited until I got sick to get it, although some people say to get it the night you board. However, the nurses in sickbay have told me that a beforehand shot does not work.
Anyway, I have been on QE2 six times and QM2 one time over the past three years. I always get seasick --usually on the second day out no matter where we are traveling-- so I plan my schedule around being ill the morning of the second day, getting the shot at noon, sleeping all afternoon, waking up, calling room service and having a wonderful burger and fries. It really hits the spot after the deep sleep and tastes so good! Plus, I feel better.
The shot did not put me in a stupor after I woke up from the nap. I also keep on a schedule of sea sick pills purchased from the purser. (They use to be free.)
Good luck and just keep in mind that a touch or bout of seasickness is part of the experience.