Thankfully, ginger has done the trick for me every time, except once when I had some bad food on the Superferry. I'm still blaming that one on the food and not my seaworthiness.I have used the scopolamine patch for seasickness and it was very effective for me. One patch lasts about 72 hours. Last I checked, you still need a prescription to get it. The patch needs to be applied several hours before boarding the boat to get a decent level of the drug in your bloodstream.
I have found over-the-counter dramamine to be equally effective when taken at least an hour before boarding the boat.
Of course, everyone is different, but this has been my experience.
I'm not sure how we got onto this topic, but here's some additional food for thought.
From DAN Divers Alert Network : DAN Takes a Look at Over-the-Counter Medications
* Anti-Motion Sickness Preparations
It's best not to self-medicate here; specific warnings regularly prohibit the use of these medications prior to consulting a physician. It's generally agreed that - at any time - recreational divers should use these medications with caution.
As with some antihistamines, these medications may typically contain meclizine hydrochloride, dimenhydrinate, diphenhydramine hydrochloride and cyclizine. Common side effects are drowsiness and fatigue. Coupled with impairment of a diver's ability to perform hazardous activities requiring mental alertness or physical coordination, these side effects will definitely not enhance the pleasure of a dive.
And a complete article specifically on motion sickness: DAN Divers Alert Network : Motion Sickness
along with a 2003 updated article: DAN Divers Alert Network : Motion Sickness - Updated 2003
And finally an article specific to Transderm Scop (the patch recommended above): DAN Divers Alert Network : Transderm Scop (The Patch)
of specific note (emphasis mine):
Transderm Scop is [the author's] personal choice for motion sickness prophylaxis, and you'll find it recommended in the U.S. Navy Diving Manual (paragraph 8-8, 1993 edition). However, certain people do suffer side effects that would make it inappropriate for them. A physician must prescribe it, and one should test it before diving. Wear a patch for at least 24 hours on dry land in a situation where the potential side effects will not cause harm. If no side effects occur, then try it out on a dive.