I am very susceptible to motion sickness. I’ve found that Bonine helps, but I absolutely must sit where I can see the horizon. Sitting forward facing also helps. I’ve done both the ferry to Anegada and a Dive BVI snorkeling trip without any issues. Recommend taking ginger snaps or mints - especially helpful when you want to get the salt water taste out of your mouth after snorkeling. My husband goes out with DiveBVI for a dive at least once a trip - and I’m perfectly content to sit on the beach by myself. So make sure she actually wants to go.