From Openwaterpedia

noun - Bathophobia (from “bathos” means depth and “phobos” means fear) is the fear of depths. Individuals with bathophobia experience anxiety when having thoughts about falling from high places or falling into objects with significant depth. These individuals fear falling or being into long, dark hallways, deep pools, lakes or wells.

Beyond the coral reef in the deeper waters, he experienced a severe bout of bathophobia.