Haena Beach State Park
In the North of the island across from the dry cave and 9 miles past Princeton you will come to a great location for snorkeling. Once you arrive here you simply need to walk down the sandy beach to the right for awhile until you reach Tunnels. Here the water is flat and the reef is a considerable ways out, but it is great snorkeling and absolutely worthwhile. The best time for snorkeling here is in the summer.
Anini Beach in the north of Kauai is also a great snorkel location. Take a right on Kalihawai Road just after the Big Bridge. Stay on Kalihawai Road for just a few seconds and then take a left on Anini Road and follow the road to the end. You can then head out to the beach with your snorkel gear. The beach is wide and large and the waves break far out, so you will simply need to head out a ways before you start snorkeling.
Lydgate State Park
In the East is Lydgate State Park and it lies just past the Wilua River sign if you are traveling from the north. However, if you are traveling from the airport then take Hwy. 56 and keep going until you pass the Wailua Golf Course. Then, simply turn right on Leho Road and then you will reach Lydgate State Park. If you are new to snorkeling then this is a great location to start out with. There are also BBQ pits, showers, picnic tables, and this is just a great place to enjoy the beach, snorkeling, and a cookout!
In the South there are several great snorkel locations off of Poipu Road. The first is Poipu Beach Park that is across from Brennecke’s Restaurant. Here, you can enter from the right of the sandy point and just continue to the water and begin your snorkeling expedition. You will enjoy all there is to see from fish to plant life and clear water.