At the last few Strictly Sail events - Pam Wall and others have discussed how well a Canada mail order drug called Stugeron (Cinnarizine) has worked for them and many others.
--- Here is a different OTC medication --
Last weekend, my wife and I took a troop of Boy Scouts and counselers/parents around Angel Island for a short sailing experience.
A couple of the folks used an OTC drug which contains bonine and it really worked great to calm the onset of mal de mer.
Available at local drug stores (CVS in our case) it's a chewable tablet used ~ 1/2 hr before sailing or even while sailing tablet.
Here is a site on the probably more expensive version http://www.bonine.com/ .
We keep the bonine on the boat from now on for anyone above 12 years old to use.