A morbid fear of the sea or, more generally, deep, large bodies of water such as lakes.
1937 George M. Gould. Anomalies and curiosities of medicine. Sydenham Publishers, New York
Thalassophobia is the fear of the view of immense spaces or uninterrupted expanses. The Emperor Heraclius, at the age of fifty-nine, had an insurmountable fear of the view of the sea; and it is said that when he crossed the Bosphorus a bridge of boats was formed, garnished on both sides with plants and trees, obscuring all view of the water over which the Emperor peacefully traversed on horseback.
The moralist Nicole, was equally a thalassophobe, and always had to close his eyes at the sight of a large sheet of water, when he was seized with trembling in all his limbs. Occasionally some accident in youth has led to an aversion to traversing large sheets of water, and there have been instances in which persons who have fallen into the water in childhood have all their lives had a terror of crossing bridges.