I have had plenty of practise photographing swimming events recently. Both of my children are members of the local swimming team (Douglas County Stingrays). Swim photography is not as easy as you may think, the splash of the water can upset the autofocus in the camera taking the point of focus away from the subject. Lighting in swimming pools can be very low and trying to freeze the action can be another real issue.
My tips are to use as long a lens as possible and try not to shoot at less than 1/500th sec, unless you are trying for the artistic motion type shot.
I've added a few football (or soccer) images from my boys weekend games just to break up the sequences.
click the image to see the video