Jesus spoke of “the sign of the prophet Jonah”  [Matt. 12 v.39-40] as being a sign of great significance. Jonah came forth alive from “the belly of the great fish”.  [Jonah 1 v.17], We believe it was a special fish, one of a kind, not a sort of whale.  The text says, “the LORD appointed a great fish to swallow up Jonah” .
We conclude that ”the great fish” was a special creature God brought into being and God saw that this special creation was in the right place at the right time. This remarkable event foreshadowed the death and resurrection of Jesus,  The time came when “the LORD spoke to the fish, and it vomited Jonah out upon dry land.” [2 v.10] – not just a damp sandy beach!
Jonah was then ready and willing to do what the LORD commanded and hastened to preach at Nineveh (3 v.3) – there is a parallel here with the disciples of Jesus.  After Jesus came out of the grave he gave them a renewed preaching commission; it was now “ … to the end of the earth” [Acts 1 v.8] and then ascended to heaven; today that commission is nearing completion, “but the labourers are few.”[Luke 10 v.2].
But there is a further lesson too –  note the impact the sudden cessation of the storm had on the sailors. With great  reluctance they had thrown Jonah overboard.  They had not wanted to do this, they “rowed hard to get back to dry land, but they could not, for the sea grew more and more tempestuous …” [1 v.13]  Initially “each cried to his god.” [v.5]  But before they threw Jonah overboard, “they called out to the LORD” to Jonah’s LORD! They said, “O LORD, let us not perish for this man’s life, and lay not on us innocent blood”   When they threw him overboard “the sea ceased from its raging.”  How awesome was this experience.  “Then the men feared the LORD exceedingly, and they offered a sacrifice to the LORD and made vows.” [v.15,16]
We are not told any more about these sailors, but surely this changed the rest of their lives. We can see a parallel here with at least one Centurion and the Roman soldiers who witnessed the awesome events at the death of Christ, one said “Truly this man was the Son of God” [Mark 15 v.38]?  Some had been aware of his power to do miracles, eg Luke 7 v.2,3.  The first Gentile convert was a Centurion! (Acts 10).
We have the testimony of the Roman historian Tacitus (c. 110-120) that Nero put to death hundreds of Christians in Rome – and that happened in AD 64!  We conclude that many Roman soldiers and not a few Centurions, when they finished their service in Israel returned home convicted of the one and only real faith – and shared that faith – truly “then they feared the LORD”  – And us?