There are two stages to seasickness;
First you think you're going to die, then you wish you were dead. - Anon
Seasickness is an unfortunate evil that many people experience when on the water. Some people are lucky and are completely unaffected, although if they are sailing with the afflicted, this usually means they have the rather more unpleasant task of caring for their companions. On sailing vessels it can be very dangerous as the crew can be decimated by the effect, until they have found their proverbial sea legs!
There are several ways to combat seasickness. The most simple are to ensure you're:
Warm - Being cold often amplifies the effects. Also, if you're happier up on deck with your eyes firmly fixed on the horizon, if you've put lots of layers on it means you don't have to go below into a stuffy, pitching cabin!
Well Rested - If you are sleepy then your body is more prone to the effects of nausea. If you are alert and active, you are less likely to dwell on feeling sick, which doesn't help!
Well Fed - Often an empty stomach will feel the effects more than a full one. I always find this hard, as the last thing you want to do when you're feeling grotty is eat, but nonetheless, being full can lessen the effect*.
Many people find fixing their eyes on the horizon is a good way to avert seasickness. I find singing, or repeating the lyrics to a song in my head helps me take my mind off it. I'm sure half the battle is a mental one! (Not that this has helped me in the past.) If you are down below, then you should be as flat as possible, and preferably asleep. It's the tricky bit in between coming down from deck and being flat that causes most people to actually be physically ill.
One of the most popular remedies, available from most chemists. There are many varieties, the most popular that I've come across being Stugeron. The thing to remember is to take them well in advance, as they will be no use if you immediately throw up! Many pills also have the unwanted side effect of making you drowsy, which is not particularly helpful if you need to act fast in a sudden squall...
An alternative to pills, the seasickness patch is worn behind the ear, but can only be prescribed by your doctor. It has the bonus of not being able to be forgotten, but I have no idea as to their effectiveness.
A simple band worn like a bracelet with a bobble that is meant to press on the relevant acupuncture point on your wrist and solve all your problems. I was as sick as a dog, but it may work for some people.
Said to be very efficient at calming the stomach. Many people swear by taking ginger tablets, or even peppermints with them, though you do have to keep them dry or they become one sticky lump...
I have tried Stugeron and it appeared to totally conquer my seasickness. Even bashing about in a Force 9 I was able to do chart work and make cups of tea in the galley. I've found my cure!
Above everything, if you're going to be seasick, get to the Leeward side (downwind).
It's bad feeling sick, it's bad being sick, but there's nothing worse than being covered in sick!
*I think this is why stodgy meals are so popular on board ship. Ship's porridge is an especially glutinous way of gluing your stomach contents together and warding off queasiness.