Where? Go north on Route 30 from Lahaina. Turn left at the stoplight after the main Kaanapali entrance. Turning right gets you to the start of the Sugar Train. This beach is correctly called Kahekili Beach.
Airport Beach is very popular with introductory SCUBA dives and it's easy to see why: convenient parking, rest rooms, fresh water showers and plenty of fish close to shore. That's why it's good for you as a snorkeler. Look out from shore: to the left is Lanai, to the right Molokai. The reef is right there in front of the beach pavillion. It extends a good way up to your right, just off the undeveloped land, but fades out quickly to the left, toward Black Rock. Because the current pulls toward Black Rock, you might like to walk up the beach to the right and drift back to the developed beach.
There are lots of colorful fish close to shore, in the rock and dead coral. They're easy to see in knee deep water as long as there isn't a lot of sand churn. Go out a little deeper to see colorful live coral. If you want a place to swim with sand, no coral, move toward Black Rock.