I tried Transderm once, during my IDC of all times!
I had a very bad reaction to the stuff. Went on it three days ahead of time to make sure I'd be OK and man did it mess me up. It actually had the reverse effect, gave me sea sickness without being on the sea. So yea, for sure try it WAY in advance and it does take 8 hours to get out of your system, its NOT instantaneous.
Ya know, I'm more of a hypochondriac about sea sickness, rather then someone who suffers from it. I've done plenty of boat diving and I usually just find a bench to lay on and close my eyes during the trip out. Usually once on the site, your so busy with things, the boat moving is inconsequential.
I honestly wish there was a better "cure" for sea sickness. Sadly, its all about getting your sea legs and once you do, you'll be ripe as rain. I have a few friends who did live-aboards and after the third day, didn't even think twice about sea sickness.
I like the Zophran idea, might try that if I can work up the gumption to hit another boat dive! heh