Shade, cover, food, and surprisingly a lack of fishing pressure. The bottoms of the boat stalls are covered with algae, which in turn attracts the shad that attract the bass. Marinas, however, can be difficult to fish effectively. There are so many cables, crossbars and tight spaces that it only rewards the very best casters. Learn to skip a bait under a dock like a stone.
Spinning tackle is easier, but baitcasting is more precise.
It just takes more practice. Sometimes I will get down on my knees to access a particular opening between a dock and a boat. Choose a lure that skips easily like a weightless soft jerkbait or a grub. If you can get into the very far reaches of a dock, then you will catch bass that other anglers will miss. Another way to score on clear water lakes is to fish at night.