If you are coming to Oahu you must go to Hanauma Bay. Even the most novice snorkler will have a good time, my 5 year old was able to see fish without the aid of a snorkel.
We have seen turtles in the past, and the veiw going down is just breath taking.
There is the minor annoyance of the movie you have to watch, but it is there for a good reason, and does teach anyone who hasn't grown up around reefs how to treat them.
The beach is clean, and the crowd is mildly controlled, not to mention there are life guards and almost NO surf.
more