The deck railings are fairly high. You are not going to fall overboard unless you try or act very very foolishly.
Many of the beaches that you have to chose from are shallow for a long way out into the water. Try to find the ones that are in protected coves then you won't need to worry too much about deep water. Get a snorkel and mask, you may be able to ease into swimming by snorkeling the shallow waters.
If you can wear the patch put it on before the cruise as suggested, but also keep it on a day or two after the cruise. Some people who are motion sensitive sometimes feel the rocking or some vertigo for a while after the cruise and the patch actually helps that go away faster.