Cambodia has very interesting Scuba Diving off the Southern Coast in the Bay of Cambodia (Bay of Thailand).  Less of the bigger fish than elsewhere, but a lot more biodiversity.

The water here is very shallow, and visibility is best at the far out islands.

Several Scuba diving shops operate from SihanoukVille, Cambodia.

Also, a few dive shops have operations right on the Islands of SihanoukVille.

Scuba Diving shops in SihanoukVille:
EcoSea Dive
Scuba Nation
Dive Shop Cambodia
Island Divers
Chez Claude
Cambodian Diving Group
Koh Rong Dive Center

AquaManu in SihanoukVille also has diving equipment, fishing equipment, and boats and boating supplies for sale.