Here's an easy beginning exercise to help develop your Cognitive/Kinesthetic Awareness and to understand the basic mechanics of motion sickness:
Stand-up, close your eyes, and have a friend push you at random intervals from any direction around you. Try to steady yourself and resist being knocked over as best you can. After a while, you might start feeling disoriented or even dizzy & nauseous (the start of motion sickness).
Take a break . . .and then try again:
Stand-up, close your eyes, and this time have your friend at 5 second intervals push from directly in front of you, and then 5 seconds later push you coming from your right side. Repeat this set over and over --Anticipate, brace yourself and counter-react to these pushes to keep from being knocked over. You shouldn't be as disoriented as you were before because you're now in control, countering these predictable, periodic and expected forces trying to push you over.
Now apply this to being on the diveboat: you see the swells coming in a regular predictable frequency (every 12 seconds for example) --Anticipate, brace yourself and counter-react to these "pushes" to keep from being knocked over. Now close your eyes and do the same thing. At the very least, with practice (and hopefully easy rhythm swells, and non-heavy, stormy or chaotic seas to start off with!), you should be able hold off the nausea to a tolerable level --and in time--
without using any medication whatsoever.
Be patient and don't give up: it takes time, concentration & effort to develop and apply this cognitive/kinesthetic technique to the seemingly complex 3-dimensional forces acting on you at sea and achieve some relief of motion sickness symptoms . . .the gist is to figure out and feel the rhythm of the swells and anticipate yours & the boat's resultant movement. . .
see also:
http://www.scubaboard.com/forums/5367954-post40.html