Went in November and did Liko Kauai Cruises. Awesome tour and the guides were knowledgeable/personable.

Our best snorkeling was tunnels beach. If you go to tunnels be sure to walk up the beach from the general parking area about 200 yards or so. The reef comes right up to the sand and goes out about 100 feet or so where it drops off dramatically. Its amazing, but the currents can be sketchy. If you don't have your own gear, visit the life guard stands. They usually have a bunch of washed up snorkeling gear.

PM if you want more details.