Good topic Adam, - one thing not really mentioned about medication is that prevents seasickness but does not cure it. If you are going to take medication, take it 1/2 an hour before you get on board. Once you are starting to feel the impact it's usually too late.
Interesting you talk about feeling ill in the water - I get seasick on boats and if I'm feeling a little queasy I feel instantly better once in the water - not that I've been snorkelling or diving in really big rolling swells, but I get surge on shore dives around Sydney all the time and it doesn't seem to bother me at all. Of course everyone is unique, but I always encourage people feeling sick on the boat to hop in the water.
The other thing that can get you trough a rough patch till you get in the water is closing your eyes for long enough for the nausea to settle down. Lying down can help but you probably don't want to do it inside.
Food can also make a difference - you probably don't want anything too acidic, greasy or spicey, so chugging back a big bottle of Orange Juice before hopping on board is probably not good as is travelling on an empty stomach.