The Dive by Sara Ochs

Breathe normally if something goes wrong.
Scuba diving instructor Cass leads her students out for their first dive off the beautiful coast of Koh Sang, Thailand's world-famous party island. It's supposed to be a life-changing experience, but things quickly spiral out of control...

Always dive with someone you trust.
By the time she gets back to the shore, one of her students is dead, another critically injured, and she knows that her idyllic life is about to be smashed to pieces on the rocks.

Don't panic if your oxygen is running out.
Someone has discovered Cass's secret, and on an island as remote as this, accidents happen. Plenty of backpackers choose to stay here forever - but some never make it home alive...