‘And he said to them, “I am a Hebrew; and I fear YHWH, the God of heaven, who has made the sea and the dry land.” '
Jonah answered in terms that they could understand. ‘Hebrew' was the description used by outsiders of Israelites. It had originally arisen because they were a nomadic people with no permanent ties to the land in which they lived. Habiru was originally a name given to landless and stateless people (e.g. Israel, both in Egypt and when they arrived in Canaan, Abraham ‘the Hebrew', an so on), and had eventually become attached to Israelites as a kind of nickname.
Jonah then explained that he reverenced and worshipped YHWH the God of heaven, Who had made the sea and the dry land. He was admitting that these rough seas could well have been the handiwork of his God.