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 bio diversity.
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:
Dive Shop Cambodia
Cambodian Diving Group
Koh Rong Dive Center
AquaManu in SihanoukVille also has diving equipment, fishing equipment, and boats and boating supplies for sale.