DJ,
I experienced almost the complete opposite. We did a 4 night cruise after 7 days of WDW, so I probably had plenty of medication in my system... I got a OTC version from Walmart that had to be taken every 4-6 hrs, so in the parks when I could tell it was wearing off I would just take the next dose, gave these to DS (I think he was 10 at the time which is why I didn't buy the daily dose ones - those said 12 or older). We weren't drowsy or anything... just could tell when it was starting to wear off after 5-6 hrs. We took it every morning & then about 2-3 times rest of the day.
When we were on the boat (4 day) I felt the waters, I didn't love sitting in Animator's Palatte (I think it may be above engines??), and basically any time I was sitting in one place (movies, shows, dinner, etc.) I could feel the "motion". I never got sick... just that queesy "I don't feel good" feeling. Again, we took the OTC every 4-6 hrs, still no drowsy, but DS & I both "felt the motion". And we had been taking the medication for a wk prior in WDW so should have had plenty in system.....
Next cruise (7 day - western) on Wednesday I realized we had forgotten to take several doses, decided to take a risk and see if we got that "funny feeling" again. (Reason I decided to try to take a break was this time neither of us complained about the "motion" and I didn't feel it every time I was sitting down.) Anyways, we didn't take any more doses of the medication and both of us were fine. Which I will admit is odd because we both get motion sickness, pretty bad. This time we didn't have any pre-cruise WDW time, so we only started taking it the morning we were leaving port. I don't know what the difference was, but we're going back in November. I will take the medication with me, because you never know, but I think it just all depends on the individual. Take a few options, the OTC walmart pills were like 100 for under $5, the daily "version" I think was similar cost. I don't think either of those were bonine or dramamine, but not sure of name... I've taken Bonine & Dramamine before, all of them seem to keep me from actually getting sick, but on the shorter cruise, there were times I wanted to get up & walk around because sitting really didn't "feel good".
Sorry so long... HTH. I think it all just depends... just be prepared.