At the beginning of the movie, we see Benjamin staring into the fish tank in his bedroom, which contains a miniature statue of a scuba diver. Later on, we see Ben actually don a scuba costume, and float down to the bottom of his family's swimming pool, where he can look up at the seemingly distant birthday party guests above. Additionally, while he's having his affair with Mrs. Robinson, he likes to drift around in the pool—a symbol for the bewildered stasis in his life. He tells his father: "[I]t's very comfortable just to drift here."
In shooting the scenes of Ben floating in the pool, Nichols said that "I always wanted to leave as much water in the frame as I could because it was about the nowhereness of Benjamin's experience." (Source)
As one film analyst describes the scuba scene:
As the underwater camera pulls away, Benjamin becomes small in the background, quite literally becoming the size of the scuba diver figurine in his fish tank. If you've ever tried to move around in water, you get the analogy. The weight of the real world is on him, and the "water pressure" is enough to drown him. (Source)
Actually, it's right after the scene where he dives into the pool in his scuba get-up that he arranges his first tryst with Mrs. Robinson. The personal isolation he feels has driven him to take up her somewhat sordid offer.