Two that come to mind...
"The Snake" - no idea how he manages to make any forward progress, as all of his body motion is side-to-side. Kinda resembles a big pink sea snake in a black speedo - and he always wears his locker key around his upper arm, so you can kinda hear it clinking against the metal plate with the locker # on it underwater.
"The Pear" - this guy has got to go at least 400#, with most of it settled in his ass. No, I'm not busting on him for his size, in fact God bless him for doing something relatively active, but he is the most rude and inconsiderate person there. Our pool gets deeper across, rather than lengthwise (e.g., lane 5 is deep, lane 1 shallow). Seems that lane 2 is the only one with the appropriate depth to buoy his girth, and God help you if you happen to be swimming there when he shows up. He thinks nothing of trying to grab your leg to get your attention while you're in the middle of a set. If you ignore his attempts, preferring to wait until you finish whatever you're currently doing, he will park his enormous ass at the end of the lane, forcing you to stop. Even if all the other lanes are full, he thinks nothing of asking you to move so that he can do his thing (which consists of strolling back and forth in the lane while staring at a HRM - the chest strap for which is completely engulfed).
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"The will to win means nothing without the will to prepare" - Juma Ikangaa http://www.litespeed.com