thalassophobia

-fear of deep and large bodies of water. (broad oceans, lakes, etc..)
-intense fear of the unkown under water. scared and uncomfortable feeling of not knowing what is under the water, how deep it is, and the darkness underneath

a person with thalassophobia experiences intense fear going into deep waters because of the unknown, (mainly gigantic sea creatures, unknown sounds, drowning, etc..)

"I am uncomfortable swimming there, i have thalassophobia"