‘And he said to them, “Take me up, and cast me forth into the sea, so will the sea be calm for you, for I know that for my sake this great tempest is on you.” '
As a prophet his reply was that, recognising that this great storm had arisen because of him (for his sake), they must throw him into the sea as a kind of appeasement offering to YHWH. Then the sea would become calm for them. he had made the decision that he was ready to die to save the ship and the crew.