A rash guard is a tight shirt surfers wear when the water is too warm for a wetsuit.  These are worn to protect the surfer’s body.  The wax on the surfboard is very sticky and will tear the skin if a rash guard is not worn.  The rash guard also protects against the sun.

Tip – if you are nervous about your apple/garmin watch falling off in the water you can wear a long sleeve rashguard to help protect it.