I have never been sea sick but been on enough boats to see that it not nice
(sorry can't help it - my dad ran a small fishing boat out of Greymouth and some of my first memories was on that little boat with a thermos of tea and a pack of ginger nuts). Now that I am all grown up and teaching medicine on boats, this subject comes up a lot.
First of all, no one is 100% sure what is the exact cause of sea sickness, as it effects some people differently than others. So the simplest explanation that works most of the time, is that there is some confusion in the brain between what the eyes and what the semi-circular canals are saying. The eyes are saying that boat is still and the outside is moving, the semi-circular is saying everything is moving all the time, the result is sea sickness - We were not built to live on the sea. Until the brain sorts out the confusion (getting your sea legs) the result is not so nice for some and death warmed up for others. To test it, spend time on a boat, come back to the shore; close your eyes and everything is moving even though your back on land - and unfortunately some sailers get land sick after spending weeks at sea. That is your brain filtering the constant motion as the new steady state.
There's also a large amount of self induced sea sickness, which contributes. Sea sickness is almost like a self for filling prophecy. "every time I get on a boat I am sick" - and every time you do, you are. It's the same when you have lunch at the same time every day - your body knows this and starts to prepare ahead of time (try it some time and miss lunch and find out the results, one grumpy, noisy tummy). When you get on a boat your stomach (brain) starts to prepare to be sick and guess what ... you are. Andy's comment is correct, toughen the F up - refuse to be sick, refuse to even entertain the possibility of being sick.
But if someone is sick, stop the smell from reaching your nose - smell bypasses most of the brain's filtering and goes almost straight to the amygdala - sometimes known as the emotional center of the brain. This is why smells can trigger such strong emotional response; if you want to sell your house quickly, have a freshly baked cookies and brewed coffee. If you want to be sick smell vomit
One of those foil lined paper bags from the supermarket rolled down will provide about 40 seconds to get to the bin before it spills the contents. A better option is a hospital Embag bag - it will provide a spill and smell proof container.
If you want to go down the medication route - read the instructions carefully, some will take 12 hours to take effect. There are as many seasickness medications as there are sailors, what works for you may not work for others so before the trip, test the medication out at least a week prior, to ensure that the medication does not cause nausea, or make you drowsy and gives you a chance to try something else. Possibly the best all around anti-sea sickness medication is Stugeron (cinnarizine), this unfortunately not available in NZ, so if you know anyone coming from the UK get them to bring some, other wise any of the ones sold here will work, the trick is finding the one that works best for you.
If seasickness is prolonged or severe then dehydration can result. This may cause electrolyte imbalance, which in the most severe cases can be life threatening. No mater how sick they are maintain adequate fluid intake, I would avoid most medications if diving, lots of reasons already explained here.
Most natural anti-sea sickness remedies just don't work and have been shown not to work - for example sea bands - their add "tested by the Royal Navy" what they left out - "and shown not to be anymore effective than a placebo". The one notable exception is ginger this has worked in alleviating some of the nausea associated with chemotherapy. It someone wants to dive I will normally give them Bundaberg Ginger Beer - it seems to have a good amount of ginger, replaces fluids, electrolytes and can settle an upset stomach. I also like the carbonated element; one of the precursors of being sick is the stomach starts to pressurize it's self prior to vomiting, by drinking a carbonated drink it helps you burb out the excess pressure.
Other non-medication methods of limiting seasickness include:
• Staying close to the centre of gravity, away from the bow
• Stay busy
• Minimising head movement
• Focusing on the horizon
• Avoiding staying in the cabin. If that is not possible, circulate fresh air.
My treatment for acute seasickness is Prochlorperazine Maleate 3mg tablets (Buccastem), it can be administered to reduce the vomiting to allow fluids to be given. It is a tablet that is placed under the lip and left to dissolve - I would not use this diving.
Remember that anti-vomiting medications should never be given for vomiting caused by alcohol or other recreational drugs.
Sorry for the length of this - but it is a subject near and dear to me