Thalassophobia is the persistent and intense fear of deep bodies of water such as the sea, oceans, pools, or lakes. Though very closely related, thalassophobia should not be confused with aquaphobia which is classified as the fear of water itself. Thalassophobia can include fear of being in deep bodies of water, fear of the vast emptiness of the sea, of sea waves, aquatic creatures, and fear of distance from land.

Source: Wikipedia