Assuming you have a car, there are places in the south. Poipu Beach park has a decent area and even makes the this top 10 list.
http://www.bestplaceshawaii.com/tips...norkeling.html
And a couple of other lists:
http://www.kauaiexplorer.com/guides/...snorkeling.php
http://www.tropicalsnorkeling.com/ka...ng-hawaii.html
The Napali coast sites are supposed to be good although I have no experience with them. The weather is pretty tricky on that coast.
I wish I could remember the name of the company who we did our tour with, they were really good.
EDIT: Found the company.
http://www.kauaiseariders.com/
I appreciate tours like this with someone who has studied the animals you are going to see and Tara has the experience. I recall her saying the week before she had Disney/Pixar our scouting locations for a movie.