Warn tourists. This should be thrown out on summary judgment. Ordering otherwise would open the doors of litigation to a wide spectrum of strange lawsuits.
People snorkel every day. You assume the risk of the activity. Expert reports, and I'll bet prior medical records, will show he had a pre-disposition to the health hazard.
Healthy people just don't fall over and die like he did. Plenty of people snorkel in Hawaii after flying and live to tell about it.