Scuba diving is a growing industry as a form of ‘nature tourism’ allowing enjoyment of the beauty of coral reef ecosystems. However, as the recreational diver population has increased, diving activities in some heavily dived sites have caused negative ecological impacts, with divers unintentionally contacting live corals and causing physical damage. In this research, we investigated the rate of divers' contacts and physical damage by observing divers' underwater behavior in Okinawa, Japan, and tested the effectiveness of pre-dive briefings as a mitigation measure. Of 105 divers observed, 7% contacted the live coral cover and caused skeletal breakage, and 91% made contact at least once but did not cause visible damage. The average contact frequency of the divers during 30 min of dive time was 0.53 times/min. The contact frequency of the divers decreased significantly after they listened to an environmental briefing among the divers who could maintain neutral buoyancy, but not in the divers who could not. This study suggests that buoyancy control training for divers may also be important for coral reef conservation in addition to environmental education.