"'You hurled me into the deep, into the very heart of the seas, and the currents swirled about me; all your waves and breakers swept over me. I said, "I have been banished from your sight; yet I will look again toward your holy temple."'" NIV translation
Jonah recalls how he was in the sea covered by the waves (another example of baptism) and called out to God. He says that he was "banished from your sight" this is a common way of describing death.
"The engulfing waters threatened me, the deep surrounded me; seaweed was wrapped around my head. To the roots of the mountains I sank down; the earth beneath barred me in forever. But you brought my life up from the pit, O Lord my God." NIV translation
This passage paints a grim picture as we can imagine Jonah sinking to the bottom of the sea and then being swallowed by a big fish. He even describes the seaweed which the fish had probably eaten. "the earth barred me in forever" is a description of the grave as at the time that was how it was described. Once again the prayer points to his death but then he says he was lifted up and praised God for doing so.
It is such a beautiful picture of Jesus' resurrection and it is also comforting to remember that we too will be resurrected to live with God unless Jesus comes back while we are still alive.
"'When my life was ebbing away, I remembered you Lord, and my prayer rose to you, to your holy temple.'" NIV translation
Now, we can see when Jonah prayed as he says it was "When my life was ebbing away". That tells us that he was dying and he knew it but he also knew where to send his appeal for mercy (God's holy temple). This can remind us that there is hope to the very end. When we have friends and relatives that reject Jesus Christ, we can take comfort in the fact that God is waiting and listening for their plea for mercy. We are to tell them that God is there waiting and trust Him to do the rest.