I don't get motion sickness all that often, but I definitely do get it from time to time. I've only gotten it once on a dive, on a long surface swim out to the mouth of Whaler's Cove in Carmel, in really choppy conditions. We probably shouldn't have been out that day at all, but I guess we were being dumb tough guys about it.
We reached the spot where we would descend, and my buddy and I started fiddling with our gauges and gizmos, and just generally looking down at our hands. Meanwhile we bobbed up and down, up and down with the waves. We were comparing compass headings and shouting at each other, and I started to feel that, "oh s--- I'm gonna barf" feeling. I stopped talking to my buddy, froze, tried to stare at the horizon, and think of puppies and bubblegum. It didn't work, and when I started heaving, my natural instinct was to bend at the waist, which put my face in the water as I exhaled, hard. It felt like it took my whole brain to override my body's desire to inhale a lungful of seawater, and to relax my stomach to let my face out of the water.
Point is, I've learned to always carry dramamine with me, and will pop one if the conditions look choppy. Antihistamines take awhile to kick in, so it's good to swallow it plenty ahead of time. The other thing I learned was, once you find yourself feeling queasy on the surface, put your reg in your mouth! It's perfectly OK to toss your cookies into it if you need to, and the reg will keep you from sucking a bunch of saltwater down into your lungs. It really doesn't take much saltwater destroy your lungs and kill you, maybe 1-2 cups / 250-500 ml. Much better to worry about giving your second stage some extra cleaning when you get home.