Although well known for its scenic beach, the snorkeling off Kailua Beach is also popular. For very experienced snorkelers, offshore of the Kailua Beach boatramp about 700m is Popoia Island (aka Flat Island) that offers some intersesting snorkeling.
To reach Kailua Beach from Waikiki, follow H1 to the Pali Highway (#61). Then, take the Pali over to the Windward side of Oahu. Stay on the Pali Highway at Kamehameha Highway the Pali Highway turns into Kalanianaole Highway. Take Kalanianaole Highway to Kailua Road. You will not need to make a turn onto Kailua Rd, just bear to the left where Kailua Road meets Kalanianaole Highway. At the triangle intersection of Kailua Road and Kuulei Road, bear to the right to stay on Kailua Road. At the second signal intersection, make a left turn to stay on Kailua Road. Go through the intersection of Kailua Road and Kawailo Road and you will be in the park. 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, Kailua Beach Park is the 11th most popular snorkel dive spot of all 23 snorkeling dives in Oahu.