If you want to discover the Andamanen Sea Ao Nang is a very good place to start.
There are many trips to do, for example the 5-Island-Tour.

This trip is headding north to Ko Daeng (snorkling, many fishes), Ko Hong (Lagoon), Ko Bileh (beautiful beach) ...

... and Paradise Island.

On the way back to Ao Nang there is another snorkling stop in front of a very little island.