Whilst I support most of the comments made do not take it for granted that you will be sea sick. A cruise ship is a much different vessel from a car ferry. I am not saying that you should not be prepared but I am saying don't expect to be sick. .... Neil.
I find Stugeron very good, but I also use the accupressure Sea Bands, although not very glamorous they do seem to work. If you are on one of the newer large ships or in the Caribbean it definitely will be far better than a cross channel ferry - mostly I find I do not need anything at all.
Whilst people can suggest what is good for them it might not be for you, so why not pop into see the practice nurse who works at your GP's medical centre.
People, including my wife use the grey wrist bands with great success, no tablets required.
My husband used to suffer from bad seasickness until he discovered Stugeron. Now he has no problems - even in a Force 10 gale in the Med a few years ago!!
Your Doctor can prescribe you Patches they work well for my granddaughter but can have some very odd side effects,and my wife uses a pill we bought on a ship,they worked well,and you can get injections on the ship if all else fails. But as Blyth says if you can try the practice nurse she may well give you the patches.
I find Stugeron works for me - wristbands do not, but perhaps I should have worn them earlier. Ginger tablets have been shown to work for some people. Take the Stugeron before you sail and top up regularly if you need to. Cruise ships often have Dramamine which is also very effective. Cruise ships are generally more comfortable than ferries. Have a lovely holiday.