MIO watches have varying levels of water resistance. Watches with 10 m of water resistance are suitable for splashes, sweat or getting caught in the rain.  Keep in mind the the depth specifed (on any watch) represents tests done in a lab and not in the ocean.  The watches have been pressure tested to that level and may not represent actual swimming results.  While you are able to use your MIO in the water, you may experience some difficulty in obtaining an accurate reading because too much moisture between the watch back plate and your wrist (like that experienced when swimming) can make picking up the ECG signal more difficult. Try to wipe excess water from the watch and your fingers before using the HR function for best performance. It is also important that you avoid depressing any of the buttons while your watch is submerged in water.