CO2 build-up is not worth blaming for snorkelling drownings. Once CO2 is above normal in body individual would simply have stronger urge-to-breathe, breathe quicker, and if breathing through snorkel becomes insufficient individual would simply lift head out of water and remove snorkel to catch breath. CO2 build-up couldn't be the 'silent killer' being suggested here; it's much too uncomfortable to ignore to reach the point of incapacitation. It's simply the victims' relative lack of experience with ocean swimming and negotiating situations such as waves and currents.

If you really want to understand how oxygen, CO2, etc., work in the body physiologically during snorkelling I highly recommend taking a freediving class (been teaching them on all the islands since 2010). As for visitors...Not a whole lot that can be done except to publish these statistics and guidance in their visitor materials and strongly suggest they snorkel in front of lifeguard towers if not experienced.