My routine to avoid " that feeling" would start the day before wheels up with Bonine in the morning. Then every morning through air travel ( usually one day but stuff does happen). Every morning for the first 3 days on the boat, at which time I'd try a day without. Usually that would do the trick for me for the rest of the trip, but if seas were ugly or we were planning a long(ish) sailing leg, I'd just dose up again for that day. ALL motion sickness remedies work best if you take/use them before you begin to feel sick.

HINT: if you are heading out of the BVI from St. Thomas, either by sail or ferry, it is wise to dose up in advance of that trip. Pillsbury Sound is a known trigger, often called " the washing machine".

Usual advice applies-- stay hydrated, watch your alcohol intake so you remain aware. Eyes out of the boat when sailing, don't be reading or knitting or below deck. Keep aware of the horizon at all times, take a turn at the wheel ( sit with the captain AT the wheel if you aren't wheel ready). Motion sickness is a physical thing, there is a dispute happening between what your inner ear tells you and what your eyes are telling you. Much better living by chemistry when you can head off that dispute.

I don't know what the BVI pharmacies have or recommend for motion sickness, but they are not hamstrung by the US FDA. Don't leave Tortola without a remedy if you know or suspect you'll suffer. Tacking up the Sir Francis Drake channel first day in a sporty breeze is not great fun if you are green.