Southeast of Honolulu and Diamond Head, the snorkeling at Kaalawai Beach is known as the best snorkeling dive site close to Waikiki. The water clarity is much better than the beaches right at Waikiki. The best section of Kaalawai Beach for snorkeling is the eastern end near Black Point.
There isn't great access to Kaalawai Beach. The easiest option is to park at the Kuilei Cliffs Beach Park lookout and then walk along the shoreline towards Black Point.
For the best snorkeling at Kaalawai Beach, try for a windless day with low surf and no rain on the days prior. To reach the best snorkeling on Oahu by air, you will need to fly into Honolulu International Airport (HNL) which is located just on the west side of Honolulu. There are direct flights into Honolulu from the US, Australia, New Zealand, American Samoa, Tahiti, Japan and Fiji. In addition, there are inter-island flights to all the main Hawaiian Islands. There is good snorkeling on Oahu year-round. But, the conditions very around the island depending upon time of year due to winds and currents. The north shore snorkeling dives are best from June through September and are usually closed out the rest of the year due to high surf. The south shore dive sites are best from November through May. The main exception is Hanauma Bay, which tends to be good year-round since it is very protected.
Overall, Kaalawai Beach is the 18th most popular snorkel dive spot of all 23 snorkeling dives in Oahu.