On most beaches around the islands the waves break over the coral shallows and then races back to the ocean through fissures or 'Avas. When the surf is running heavy these become rapid rivers and you can get swept out to the deep ocean in a few seconds.
Many swimmers get stranded offshore and every year some don't make it back.
Never fight a rip tide, swim accross it, usually parallel to the beach, then head for shore when you have crossed the current. The bigger waves can really pound you onto the coral and you can easily get scrapes and cuts from it.