Always carry one. However, on a recent liveaboard (only boat in the middle of nowhere) diving early morning, the dinghy boats could not see it (7'er) due to the sun behind it and serious swells. They "thought" I must still be down diving. I could not see them but mirrored and whistled to the main boat which got smaller and smaller due to swift current. Though we were told to never swim back, my only chance of being seen was to kick my A** off to get back in range of the main boat and keeping that SMB vertical was a maximum challenge. They finally found me after 2 hours. So...a radio will now also be part of my gear.