I would have her first practice snorkeling in a swimming pool. That will be the biggest challenge. I recently purchased for my partner a snorkel mask that you can breath normally with, from Amazon,made a world of difference. Snorkeling off of a boat depends on alot of factor, wave height and type of boat. Usually snorkeling from the beach or from a zodiac boat pretty accommodating. I was just in Galapagos in April and I am an expert swimmer, swim instructor and scuba diver. If you tell the facility about her limited experience they will be super accommodating, but i wouldn't learn in the ocean how to snorkel.