One look at this pristine, sandy beach shows why Hanauma Bay is one of the most popular destinations for swimming, snorkeling and scuba diving along the windward coast of O’ahu just 20 minutes from Honolulu. The average temperature for the aquamarine water is 74°. In the background is the Ko’olau Mountain Range. The impressive mount was formed by a series of volcanic eruptions. The last one occurred about 40,000 years ago.