I scuba dive so go on boats a lot. I also suffer badly with motion sickness - I can even get sick underwater during a dive if there is a strong swell!
The wrist bands work brilliantly for me but only if they are placed in the correct position, which is one exact spot. I've seen people wearing them inches away from where they should be, and then complaining that they didn't work. I wear them under my wet suit while I'm diving.
If the sea is really rough I also take 300 micrograms of hyoscine, and keep my eyes fixed on the horizon, which also helps a lot. It's a long time since I threw up on a boat, except one day, in rough seas, when I took the hyoscine but forgot the wrist bands.
Gill