An irrational fear of depths, deep water, or diving, or a fear of any activities involving deep water.
Greek bathos 'depth' + phobos 'fear'. It follows the standard pattern for naming phobias in psychology that emerged in the 19th century.
Bathophobia is different from aquaphobia (fear of water itself)—someone with bathophobia might be fine swimming in a pool but terrified by the thought of deep ocean water where they can't see the bottom!
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