Seasickness Medication, Yes
Maybe you need to point a gun at 'em and say swallow.
I've had several less-than perfect "offshore" cruises because some of the guests didn't take their meds. Once in a Gulfstar 54, we never even got out the ICW into the Atlantic before 3 out of 6 guests started barfing. The only revenge was to keep eating really greasy fried chicken before their eyes... Much more recently, With plenty of warnings from the skipper, a few guests ignored the advise to such an extent that the aft deck became noxiously uninhabitable when taking on the Gulf of Alaska on the rear quarter in 30 knot breeze. You know the motion: a kind of rolling corkscrew with a bounce... The blessed breeze served to return their offerings, at least, and I could swear the orcas winked.