The patch contains a drug called scopolamine which is related to belladonna. It acts as an anticholonergic, meaning that it affects the reception of certain chemicals in your nervous system.
Dry mouth is by far the most common side effect. Drowsiness, pupil dilation, decreased gut motility (sometimes leading to constipation), urinary retention (a potential side effect that is bad for folks with already enlarged prostates,) and increased heart rate are also fairly common and are directly related to the action of the drug. A fair number of people also experience a withdrawl syndrome after the patch is removed including headache and dizziness.
Meclizine (Bonine)has far fewer side effects and a dose of 25 mg a day is effective in the majority of people. If you would like to try the patch, I would agree with giving it a "dry run" on land before you cruise.
There are some great posts written by a pharmacist that discuss the risks and benefits of all the available meds for motion sickness. I am a physician, and because of the side effect profiles, I always choose meclizine over scopolamine.
I hope this is helpful.
Renee