"When David's doubts clogged up his faith he did not give up and let the ship run, as we say, before the storm. Instead of doubting if God loved him he communed with his own heart and his spirit searched diligently: "In the day of my trouble I sought the Lord" Psalm 77:2. A person should no more sit down and be content in his unresolved doubt than one who thinks he smells fire in his house would go to bed and sleep. He will look in every room and corner until he is satisfied that everything is safe.
The doubting soul is much more afraid of waking with hell-fire about it; but a soul under the power of unbelief is falsely secure and careless. Because the old world did not believe in an impending flood, the men settled down into a lethargic refusal to consider God's warning. And water reached their windows before they had the means to escape.
In spite of his doubts the true believer leans on and desires still to cling to Christ. While Peter's feet were faltering beneath the water he was lifting up prayer to Christ; and this proved the truth of his faith. Although Jonah had many fears, yet even in these his faith had some little secret hold on God: "Then I said, I am cast out of thy sight; yet I will look again toward thy holy temple" Jonah 2:4. "When my soul fainted within me I remembered the Lord" (v. 7). And David also, though he could not get rid of all the fears, which came in through his weak faith, as water into a leaking ship, raised a firm hand and cut them off. "What time I am afraid, I will trust in thee" Psalm 56:3.
The weak Christian's doubting is like the wavering of a ship at anchor-- he is moved, yet not removed from his hold on Christ; but the unbeliever's doubting is like the wavering of a wave which has nothing to anchor it and is completely at the mercy of the wind. "But let him ask in faith, nothing wavering. “For the person that wavers is like a wave of the sea driven by the wind and tossed about," James 1:6.
Quoted material from, ”The Christian in Complete Armour Daily Readings in Spiritual Warfare” by Gurnall and James S Bell. http://www.moodypublishers.com/pub_productDetail.aspx?id=41823&pid=53617