Thanks for your questions, Paul. It is a good one, asked quite frequently.
SeaBands (and similar accupressure appliances) allegedly prevent motion sickness/seasickness by applying pressure to a pressure point in the wrist, inhibiting the sensitivity to vestibular (inner ear) disturbances. Many of the claims to the effectiveness of SeaBands are anecdotal; there is no good, definitive clinical study to support this claim. My personal reflection on the claims of effectiveness: if you believe that the SeaBands will work, they probably will (placebo effect).
For my recommendations, see my response to hdawson in this message board. I personally use a half-dose of meclizine (Bonine, etc.), because it works for me, but so did a half-dose of dimenhydrinate (Dramamine). Find something that does work for you, and use it.
Hope this answers your original question, Paul. Happy cruising!
John Burns, R.Ph.