For a wooden dock to rot all you need is time. It's bound to happen sooner or later, it's just a matter of when. Why does this happen?
When moisture is present in wood, it leads to fungi growth. Fungi growth then causes the wood's fibers to deteriorate. Moral of the story is when water sits on unprotected wood for a long period of time, it begins to rot.
How to Detect it On Your Dock
You'll want to make sure you keep an eye on your dock for any signs of wood rot.
If the wood is discolored, that is a clear sign of rot that you're able to see with just your eyes. If the wood feels soft when you press on it, that is another sign. When you go to do this, take a screwdriver with you. Press on the wood with it, if it is soft, brittle or crumbles those are just more signs of rot.
How to Prevent it
If you have a wooden dock, the best thing you can do to prevent wood rot is taking great care of your dock.
Pressure wash it once a year, sand down any splinters that pop up, seal it with a marine grade seal, prevent ice and snow from building up on it in the winter by scooping it as you would your driveway and repair it whenever you see trouble.
Your dock is an investment, be sure to take care of it in order to get as many fun years as you can for your family.