When Jonah was thrown into the sea the Divine hand picked him up and carried him delicately that he might not perish or drown. God took him and placed him inside the whale to preserve him safe there.
God had “prepared a great fish to swallow Jonah" (Jon. 1:17). He did not prepare it to destroy him but to preserve him. The great fish was not a punishment but a shelter. Jonah was safer and more comfortable in the belly of the whale than if he were still in the ship struggling against the waves and the sea, and against fatigue, cold and wind.
This whale was sent by God to carry out the Divine will entrusted to it.
It did not have authority to eat Jonah nor to secrete digestive enzymes onto him to absorb him. No, but rather it swallowed him in order to take him into its inner bosom and keep him safe there until he would approach his destination. The whale was a free means of transport by which Jonah reached the nearest point to his embarkation stop.