Insert a fresh tampon right before you get in the water, that way it has full absorbency for the swim. Pick the absorbency that matches your flow that day, light for lighter days, regular or super for heavier ones. Insert according to package instructions until comfortable and not felt internally, a compact applicator like the one on Rael's Organic Cotton Tampons can make that easier. Tuck the string so it stays put inside your swimsuit. Swim as normal. When you're done, dry off and change to a fresh tampon or pad right away, don't sit around in wet swimwear with a used tampon. Period swimwear works as backup.