You rarely get seasick these days - the new big modern ships have such good stabablisers you don't feel it. If you are going with a older smaller tug-boat like FredOlsen or Thomson then it is a different answer then I'm afraid. There is a brand called Bonine that is handed out for free, by the ships I have been on.
If you are cruising I hope you have discovered the most excellant site boards.cruisecritic.com - make sure it is .com and not .co.uk. Scroll down to the link for your appropriate cruise line, and scroll down further to the Ports of Call section to find out what to do in each port thus avoiding paying for the overpriced ships excursions.