Ginger is a great preventative. I take ginger chews/candy from Trader Joe's, but also just drink ginger ale if I'm feeling a little "off". If you have HIA, it's easy to stop by a bar and ask for a can of ginger ale or two, and keep them in your fridge in case you need them later.
I also bring generic meclazine, but only use it if I really need it. Luckily I'm not very prone to seasickness, just feel a little "off" on the first sea day. The rest of the week I'm fine.