Meclizine
"I learned to sail keelboats 8 years ago.  I love it, but am prone to seasickness. I tried ginger initially but that was ineffective.  Then a friend suggested Bonine and I have used it ever since.  I don't feel drowsy while sailing, but on the drive home after sailing and all the next day I can be very drowsy.  To combat the drowsiness I tried reducing my dose, I now take 1/3-1/2 a tablet each time I go out (once or twice a week), increasing the dose up to 1-1/2 tablets if I expect rough seas.  Over the 7 years of twice a week use, I have only had trouble twice in stormy 6 ft seas.  Highly recommend, but figure out your best dose.   I have taken it for up to 10 days continuously without ill effects."