Ginger snaps are really good. Eat them as soon as you start feeling queasy. Also, SLOWLY sip a glass of water with your ginger. If you gulp it I promise you WILL puke.
Stay in the breeze, and don't read a book or go below decks. Look at the horizon, not your feet. If you can, stay in the water as long as possible: first on in, last one out. If you're getting REALLY sick, pull on your wetsuit and jump in, assuming the boat is anchored. Float on your back and don't get out until its time to leave. If you feel better and want to make the second dive, have someone hand down your gear. DON'T GET IN THE BOAT. (Trust me, I've been there before)
Also, Scop patches work great, that's what I use in addition to ginger to keep it down. Half a patch works for me, but it depends on your size and weight.