One of the best places to snorkel is called Tunnels. It is up, past Princeville; about 1/2 way to Ke'e beach, which is at the very end of the road. Tunnels has lots of reef and is great for snorkeling;EXCEPT in the winter with high tide. Because there is all of the reefs, you can get caught up on the reef if it is in the winter, because you get higher tides on the north end of Kauai in the winter. Tunnels has a super large beach and lots of people, but not a lot of places to park. Many peopl drive by it. It is right before the dry cave. Have fun!!

Some good info and tips at: http://vacation.away.com/hawaii/travel-v2-cid362299-kauai-cid13-snorkeling-vacations.html