There's always shore diving! ;-)
But seriously, i found that eating well before going on the boat to dive, i tend to not get sea sick... if I'm on an empty stomach, sometimes I get sea sick... don't know why, and it usually happens during the surface interval between dives. Once i'm back in the water i'm fine though.
If there is a very bad swell, and the waves are really choppy, that can certainly trigger it for me too. I just try to close my eyes or focus on a point on the boat... if that doesn't work, like someone else mentioned, just do what you need to do, and typically feel better right after that.