Today I used my birthday gift from my wonderful wife for the first time. The H2O Audio Interval system for swimming. I love ’em. Some experimenting with the earbuds are still required. The system comes with extra size earbuds. I used the ‘S’ and they worked okay. They came off occasionally. Wearing your swim cap over them is the ideal fit for securing the buds. However I also ordered the pro buds with a wider assortment of sizes and designs to choose from.