What the Bible says about Unbelieving Attitude
(From Forerunner Commentary)
Toiling for long hours trying to catch fish yet without results is especially exhausting and discouraging. Peter points out the obvious, but out of respect adds, "At Your word I will let down the net." Note that Jesus' command is for Peter to let down his "nets" (plural), yet he replies with "net" (singular). His obedience is half-hearted. Not completely understanding God's power in Jesus, he probably figures the result would be the same as his earlier lack of success. His unbelieving attitude exposes itself in poor-quality obedience. At this early stage, Peter is still learning about the power of the Creator to command His creation (John 1:3; Colossians 1:16).
Martin G. Collins
The Miracles of Jesus Christ: The Great Catch of Fish