Try not to get wet.
Taking both lenses would be a good idea. Is your 18-55mm the IS version? I had that lens as well. I always thought it seemed a little sharper than my 55-250IS. But when shooting around 55mm, the 18-55's F5.6 max aperture allows only half the light compared to the 55-250's max of F4.
If possible, use RAW instead of JPEG (or use both) so you can adjust the white balance easier if the color doesn't turn out the way you expect. I'm not sure how much it'll affect your burst rate, so you might want to try a burst in JPEG versus RAW to see what's acceptable speed for you.
Check your first few shots to see how it looks. Zoom in. I've thought alot of photos looked sharp until I zoomed in. Some photos will look better than others, and the camera will display what settings were used. (If they look great, you may even want to switch manual mode with those settings.)
Your camera has multiple focus modes, like 1-shot and AI Servo. The AI Servo is supposed to track a target, but depending on the camera/lens/lighting/subject your mileage will vary. 1-shot is the default mode where it focuses once. I usually use only the center focus point with 1-shot because my camera will randomly pick which of my 9-points to use. In the end, you might even need to use manual focus, which can be hard to tell whats in focus through the viewfinder. If you know where the subject will be, you can prefocus in live view and then wait for the swimmer to pass that predetermined point.