The Kona side of the Big Island has the most extensive coral reefs in the main islands. "Two-step" just outside of Pu'uohonua o Honaunau has an easy entry and has an extensive shallow reef top. It is frequented by turtles on the South side (towards the Place of Refuge) and Hawai'ian spinner dolphins frequently use the bay as a resting spot (they are unfortunately chased and harassed by swimmers).
Alula Beach on the South Side of Honokohau Harbor is another excellent snorkeling spot, especially if you are a little more comfortable with snorkeling. There is an extensive reef top in 20-30' of water that is the home to large schools of fish, including pyramid butterflyfish, snappers, and Heller's Barracuda. Turtles are also common here. This is easy to snorkel / freedive and is such a nice location that most of the dive and snorkel boats will use it as one of their stops, even though it is literally only 100 yards from the harbor entrance and is easily accessed from land.
The reefs near the Captain Cook monument in Kealakekua Bay are excellent, but are generally accessed by boat or kayak, although there is a (long and hot) trail that can also be taken to them.
If you want an exceptional experience, the manta ray night dive / snorkel is, if the animals show up (which they generally do), is amazing. When certified SCUBA divers visit, I insist they do the dive.