Won't I be seasick?
First Things First.
Sea-Sickness is just a form of motion-sickness, so anything here really applies to any form of motion sickness, sea-sickness, car-sickness, bus-sickness or space-travel. They're all the same in principle.
The next thing to say is that for most people, motion sickness can be avoided, reduced or eliminated in most circumstances, but you probably want to be prepared.
What Causes Motion Sickness?
Opinions vary a bit on this, probably depending on who is thinking about it and in what detail.
So I'll give you my opinion. I've told it to puzzled experts and usually get a reply like "Oh, I hadn't thought of it like that. I think you're on to something". So here goes:
My opinion is that unexpected and unfamiliar motion, which upsets the fluids in our inner ear responsible for our balance, makes our brain try to understand why everything is moving about in ways it can't understand. The answer our brain comes up with is "It Must Be Something I Ate". So it tries to deal with that in the only way it knows how, and tries to get rid of whatever it was. It's probably wrong, of course, but the basic animal bits of our brains won't listed to any other explanation.
If that isn't what causes it, it's as convenient a substitute explanation as any other I've heard. It does for me.
How Can I Avoid It?
For a few people, they simply appear unable to avoid it, though some love sailing enough that they'll put up with it for a while until the problem passes, which it usually does.
Most of us can reduce the risk, and can deal with the problem if it occurs, but there are a few problems, which I'll try to deal with.
Preventative
My wife, Sue, sometimes suffers motion-sickness. Her strategy is to take a dose of a sea sickness remedy 24 hours before we go sailing, and then another a couple of hours before. Sue, along with many others, uses Stugeron(R) (cinnarizine). It tends not to cause drowsiness and it isn't unpalatable ... which is important as we'll see later.
There are some acupressure and acupuncture type methods that work for some people. One in particular works well for most people. These again should be first used well before the motion starts. A couple of hours is likely fine in this case. See Sea Bands and Relief Bands It's probably worth noting that these two product work on nausea generally, not just motion sickness. Sue uses the latter if she starts to feel bad.
Avoid the brain becoming confused by the motion. Obvious, but not so easy. Learning to absorb the motion with the legs helps a huge amount and is probably a large part of the reason why most people get over sea-sickness after a while on board. Look at the horizon. You're brain will then recognise that the world is OK and you're OK, it's just the thing you're standing on that's wobbly.
Keep warm. Eat normally.
Dealing With It
If you have a Relief Band and you're not yet wearing it, wear it now. More later.
Hopefully you won't have to if the preventative methods work, but I doubt that anyone it totally immune to motion sickness, and we've probably all had days where we think "I wish now I'd taken a pill".
If you try to take a motion-sickness pill once the symptoms start, it's very likely that the very action of taking the pill will "push you over the edge".
So what to do?
Absorb the motion and look at the Horizon as for preventative above. Sometimes you may feel bad because you haven't been doing these things and you'll be surprised how much they can help.
Dissolve a motion-sickness pill under your tongue. Let it dissolve there slowly. Note this is why your pills must be palatable. If they taste disgusting, and some do, this action alone will probably make you throw up. Find out long before you feel the need to do this whether or not your chosen pills are OK.
Try to keep in an area of minimal movement of the boat (or car, or space ship). This is almost always close to the centre.
Or, try lying down in a bunk. You will almost certainly find it's much more comfortable, though it can take a few minutes. Surprisingly, closing your eyes can help. The difficulty with this particular thing is that you have to get to the bunk before you can lie in it and in a bucking boat it can seem a long, long way and there may no longer be a horizon.
Keep warm. Cold makes things worse.
Eat something even though you really won't feel like it. Dry biscuits work well. Tinned pineapple is popular as "it tastes the same coming up as it did going down".
If you throw up, it's probably best into a bucket and then empty the bucket overboard. If you have to throw up over the side, remember downwind the leeward side or direction, but remember also that this is potentially very dangerous. Make darned sure you stay on board the boat. If conditions are rough, hopefully the skipper will already have made you put on a harness and life-jacket, and made you attach yourself with a safety line. If not, remind the skipper of that.
Relief Bands
I'll tell you a story. I was sailing on a tall-ship and one of the crew had said they "swore by their relief band". I take these kinds of things with a pinch of salt. We'll see if it gets rough!
Well, it did get rough. Out amongst the Scottish island, F8, sea coming over the decks, not an easy ride.
"Where's Jean?" I asked one of the other crew.
"Oh, she's feeling sea-sick and has gone below ... but she'll be back in a few minutes once her Relief Band starts to take effect, you'll see".
And She Was!
I did some research into their principles of operation and into other people's experiences with them and immediately bought one for Sue. She doesn't use it often, partly because it gives a pricking sensation in the wrist, but when she feels the need, she wears it and so far it has always done the trick.
I'm afraid I can't tell you if sea-bands work as well. They may.
One last thing.
It's been said that there are two stages to sea-sickness. First you think you're going to die; Then you wish you had.
It can be managed ... don't try to be a hero, that's just dumb ... be prepared and manage it.