Boat docks are some of the best forms of cover, but not all boat docks offer good fishing. In some places boat docks are the only cover there is for bass. Docks also provide shade. Many people fish boat docks by flipping or pitching a Texas rigged worm. Pitch right against the posts, specially corner posts. Just shake your Texas rigged worm at the bottom of the post. Pitch or skip your bait in the shade under the dock, as far back as you can get. This often works very well. Sometimes bass are suspended and do not bite these bottom bouncing lures. The bass may be very tight to the cover but will not go down to the lure. When this happens, try these ideas.

The best docks are old and wooden and have many cross braces under water. Usually the older docks are the better docks. They've had more time for debris to build up beneath them. They've also had more time to allow algae buildup on the structure itself, whether it is styrofoam floats or rigid dock supports. This enhances the whole food chain, attracting bait fish which brings in both the bluegill and bass.
Very good and silent boat control is needed to get close but not spook the fish.
Ease the boat to within accurate pitching distance.
If bass do not bite your worm or lizard on the bottom, they may be suspended. Use a jerk bait, crank bait or spinner bait that runs where you can just barely see it. Get very close to the dock. This can also work on floating docks.
Work lures with treble hooks just deep enough so they disappear but keep them just outside the posts. Get them close. Use an erratic retrieve.
Use a spinner bait just outside the posts. As it goes by a post, kill it and let it sink a little. Bass often hit it as it sinks. Also pitch your spinner bait under the dock as far back as you can get it. Try different running depths.
A lot of times there are giveaway signs indicating whether a boat dock is enhanced for fish. Fishing chairs, rod holders, or night lights indicate there may be brush piles placed under the dock and a pretty good fishing location for you to try. These brush piles often hold more bass than the dock itself.
Work different depths. Check both the shallow postsand the deep ones until you find a pattern.