A wet exit is one of the first skills a kayaker learns: how to get out of the boat if you capsize. Many people get disoriented and panic when they are upside down in the water, so it is important to learn and practice wet exits when you first start kayaking. If you need to, practice it several times in a pool until you can do it without panicking. Here is a good article at paddling.net, excerpted from Derek Hutchinson's sea kayaking book.
http://www.paddling.net/guidelines/showArticle.html?77
Don't let the sea kayaking thing fool you, you will probably do more wet exits in a whitewater boat that you would ever do in a sea kayak. Whitewater spray skirts are often tighter on the boat and harder to remove than touring skirts, so it is important to practice this in your whitewater boat if you have never done it before.
Why did we name this site wetexit?
It's a slightly different interpretation of the term. Think about getting out of the office and onto the water. Think about taking a relaxing trip along a coastline or an exciting trip down a whitewater river. That's our wetexit.