You can often avoid seasickness by staying busy and keeping your mind occupied. Any activity that will keep you above decks and focus your mind on anything other than the swaying environment will help. Staying in fresh air instead of in a stuffy cabin may help.
Take deep breaths and drink plenty of water. The worst thing that you can do is go below decks with no land or horizon to look at.
Reading or staring at an object will assuredly bring on the affects of seasickness. Keep your senses, particularly your eyes, working flat out interpreting the motion of the boat and the waves.
Find a haven on the boat where the motion is at its minimum and which allows your eyes to gaze at the horizon. On a large ship try and face forward. Your peripheral vision is an important factor keep it out on the horizon but do not visually lock on to it.
Let you brain adjust to this unstable environment by allowing the horizon to act as a true point of reference.
If you can, try and eat lightly and avoid fatty or spicy foods. Try to stay warm, relaxed and comfortable. Try to sleep at the appropriate time and allow your brain to recover. Spending valuable leisure time in bed isn't fun, but a prone position could alleviate some of your symptoms. If possible try not to lie down in your cabin, instead find a deck chair and get some fresh air as well.
There are certain remedies that may help some sufferers and these are discussed in the next section. Medication must be taken hours before you travel and not during the onset of symptoms. Herbal remedies such as ginger are reported to have a beneficial effect. There is some evidence in the medical papers section which supports this. Some non pharmaceutical aids such as pressure bands are claimed to help.
Try choosing the type of boat, where you sail and what season you travel carefully. Modern cruise ships are equipped with stabilizers that eliminate much of the motion responsible for seasickness. Bigger is best, if your boat is a mega-liner it may not pitch and roll quite so much as a smaller craft.
If you have a history of motion sickness do not book an inside cabin, it will begin to resemble a movable coffin!
Remember - if your eyes see what your ears are feeling, you will certainly have a better chance of avoiding the worst effects of seasickness.