A few more points:
If you feel sick, don't puke in the head. Heads can easily get clogged and they need to be kept clean and working for the other crew.
Take several gallon size ziplock plastic bags and keep them on you and handy when you feel the urge. They can also be sealed and washed out and reused if needed.
Best is to puke outside, over the LEE side, not the windward side! I hope the reason is obvious.
Fresh air helps most people. Make sure the boat has good ventilation if possible.
Diesel fumes nauseate most. Be careful and avoid fuel spills (common during the last minute rush to depart).
If possible, spend one or more nights on the boat prior to departure. Try to get your inner ear used to the motion on water, especially by sleeping on board. The ocean conditions may be much rougher, but I believe having your inner ear accustomed to some boat motion prior to the voyage will help prevent early onset sickness.
Stay busy or help run the boat, such taking a turn at the wheel or tiller or tending the sheets. Avoid reading or looking down at things like charts. Try to keep your head up, not looking down!
If you feel the urge to puke, let it happen. As unpleasant as it is, you might feel completely better in minutes as soon as you have.
Don't drink alcohol the night before your voyage, if you are concerned about getting seasick.
Avoid greasy foods.
Remember, the Admiral, Lord Nelson, the greatest hero of the British navy was prone and known to get seasick.