I have been on several ocean cruises and I know from experience that I am prone to seasickness. I have previously tried using crystallized ginger, Dramamine, acupressure wristbands, etc., without success.
On my most recent cruise I got a prescription for Scopolamine Patches. These had been recommended to me by several experienced sailors. It is commonly known as 'Transderm Scop' or 'The Travel Patch', and is dispensed by placing a small adhesive patch behind your ear. You should attempt to apply the patch before you feel any motion sickness symptoms, and each patch lasts for 3 days.
During my cruise the patch worked great. I did not experience any seasickness and the only possibly side effect which I might have experienced was being thirsty. I used 4 patches during my 14 day cruise. The patches are relatively expensive, but are worth it.
Unfortunately, I had a very bad experience when the cruise was over after I took the patch off. I experienced extreme dizziness (Vertigo) and nausea for several days. I could only get out of bed for a few minutes and then had to return. I found some articles about Scopolamine withdrawal and found relief by self medicating with meclizine (Dramamine). It seems that this type of side effect occurs sometimes when the patch is used for more than 3 consecutive days.
Despite the unpleasant after effects, I will probably use Transderm Scop again.
Update On Aug 21, 2010: I recently bought some Transderm-V from Universaldrugstore.com. Transderm-V is the Canadian registered name, it is the same strength, also made by Novartis and does not require a prescription. The net cost was cheaper to get it online than it was to buy it with my medical insurance (and pay the copay). Fast shipping, too! I am willing to deal with possible rebound effects on my upcoming cruise because I hate being seasick.