Hin Daeng and Hin Muang are pinnacles that rise up 70m from the sea bed and barely break the surface, sightings of whale sharks and manta rays are common (not suitable for beginners).
Koh Rok Nok There are multiple sites around the islands that offer excellent visability and rich marine life.
The Phi Phi Islands are home to so many sites you could dive there every day for 2 weeks at a different site each dive. You can rent equipment and dive from the beach in some places or even just snorkel.
Bamboo Island is part of the Phi Phi group. Many snorkelling tours visit daily to experience the beautiful coral gardens. Sites tend to be quite shallow which is great for inexperienced divers.
Koh Dam is two islands joined together by a reef. Very close to Railey and Ao Nang, this is a good spot if you don't want to go very far.