Scopolomine, big time. Eat bland carbs, both night before and on the boat. Think unseasoned mashed potatoes and oatmeal. Do NOT avoid eating the morning of. Not only will your stomach be more unhappier, but trust me, it's better to have something in your stomach if you do feel the need; otherwise you just wretch miserably.
Stay hydrated but while on the boat, don't drink lots of water. Get your fluids in more stomach friendly formats like ginger ale and herbal tea. If you're prone to seasickness (and I am), drinking that lots of recommended water pretty much ensures you're going to be dehydrated after you end up recycling that water straight back overboard.
Minimize the amount of concentration you need on the boat. Try to find one where your gear is pretty much assembled before you leave and then bungeed, and hopefully one where they swap the tanks. You still need your basic checks, but the worst I get is when the boat is anchored and everyone's getting their stuff together and I'm needing to do extensive things with my gear. This is not a time to read dive magazines or check logbooks.
If you have an OTC remedy that seems to work well without debilitating side effects (like Dramamine or Bonine), go chewable (dissolves faster), take one the night before, and one the morning of as well as the patch.
Avoid the head if at all possible. If you have a wetsuit, warm it. Don't make yourself uncomfortable, but I've found the absolute worst place to be on a boat is in the head. You're being bounced around in a small cramped space, trying to stabilize, and it's usually in the worst part of the boat.
Find the source of the diesel fumes and stay AWAY from them. I've felt wretched while STANDING in 2' of water because I was behind a boat that was having engine trouble and was taking a face full of fumes.
Try not to be the first one up. This would kill me too. I try to be the first off, last up. You may want to do a shallower dive (providing the stuff to see is multi-level) to extend your bottom time, and contemplate things like a bigger tank and/or nitrox, depending on what's limiting you.
If you're feeling a bit queasy before you go off the boat, if you're diving someplace that isn't a wall and you can touch stuff or find a sandy area (like Monterey), sit on the bottom for a couple minutes. While you will feel better just entering the water, further stabilizing your inner ear by sitting on something that DOESN'T MOVE is a huge help when you come up. If you are incapable of diving, you're incapable of diving, but if you just feel exhausted and miserable, DO THE DIVE. You will feel so much better. On a 3 day liveaboard in the Channel Islands, I did 6 dives in one day because it was SIX HOURS off the boat.
Can you tell I'm a veteran of this stuff? Seasickness sucks, but with a combination of techniques, you can either get rid of it entirely, or at least, keep it from ruining your day. Though I'd get seasick in a swimming pool, with these methods, I can be either pain-free for most of a trip, or at least, remain functionally sick. I may not feel great, but I can hold a conversation that doesn't consist of "kill me" and assemble my gear.
Oh, and if you run into the sort of people that don't get seasick but think it's funny to show you a chili dog and watch you get sick, chum for whites and push them overboard, or barf in their dive bags.
