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On the straight on shots (fly and breast), take a moment to position yourself precisely in front of the swimmer. A small detail, but it makes a huge difference. You are a little off to the right and it completely destroys the symmetry of the stroke. If you are shooting from the side (free and, for variety, breast and fly), make it as close to 45 degrees as you can. And as another poster noted, you need to get lower. Too high and you get no strong eye contact or expression.
Once you have the positions nailed, swimming is just about timing. Freestyle is probably the trickiest because the arm can confuse autofocus on some cameras, but when you catch the arm high with water streaming off the fingers, it is one of the most dramatic. Practice on the long races, when they settle into a smooth rhythm of stroke and breathing. Except for really young swimmers, short free races are hopeless: too much splash and the older swimmers don't have to breathe except at the turn.
Longer lenses will give you better facial expression and blur the junk on the deck. I love the 300 for swim.