I'm prone to seasickness, so I've read as much as I can on the subject. 
  Turns out NASA has studied it extensively because space sickness is 
the same thing, and they've found out one really interesting thing.  I 
read that what happens is that when your inner ear is getting really 
stressed by all the heaving around, your stomach basically goes into 
fibrillation, falling out of its normal steady, regular rhythm and 
getting wildly irregular.  According to NASA, it's far less likely to do 
that if it's got something to work on, ie food. Best is something simple 
to digest-- low fat, low protein, low acid.

I've found that if I make sure to have something in my stomach when I 
get on the boat before it gets moving and keep nibbling throughout the 
voyage, I do much, much, much better.  I like saltines, so I bring a 
good-sized bag of them and just munch away.  That's really made a huge 
difference for me.

Supposedly, what causes the problem is the contradiction between what 
your inner ear is sensing about the motion of your body and what your 
eyes are seeing, so you want to minimize that contrast.  So stay on 
deck.  I think the bow is far better than the stern, too, largely 
because of the fumes but also because when you see where you're going, 
it's somehow easier for your brain to deal with the bouncing up and 
down.  And the cold fresh air battering my face also somehow contributes 
to keeping me on an even keel, so to speak...

If you do start to feel a little queasy, the very worst thing you can do 
is what we instinctively want to do, which is lie down or sit down and 
bury your head in your arms, or go below where you can't see the 
horizon.  If you do that, you're guaranteed to get actively sick.

I also take Dramamine (if that makes you too sleepy, there's another 
similar over-the-counter one that I think is called Bonine, which is 
less of a problem for some people), and I use those little wristbands 
you can get in the drugstore with little plastic nobs that push gently 
on a pressure point in your wrist that's supposed to help.  I have NOT 
experimented to find out if this really does help or not. :-)

The problem with pelagic birding is, though, when you look through binos 
at a bird on the water or in the air, you're creating a really big 
contrast between your body heaving up and down and your eyes seeing a 
relatively steady image.  So far, I've not found any amount of Saltines 
or Dramamine or anything else that will let me use my binoculars on a 
heaving boat for more than quick glimpses without starting to get into
trouble.  So the world of fulmars and shearwaters and the like is pretty 
much closed to me, unless somebody offers a pelagic on something the 
size of the Queen Elizabeth...