Thailand has diving on the Andaman Sea as well as the Gulf of Thailand. You can also get liveaboards up into Myanmar out of Phuket. Thailand is a great place to visit with lots of cheap flights and cheap hotels if you are looking for a cheap holiday scuba diving adventure.
Cambodia Still the new place to go scuba diving, lots of adventure and your boat most likely will be the only boat on the site. You will find lots of schools of fish, and big coral gardens for the snorkels as well. The live-a-boards to the Koh Tang islands are the best deals with big fish and deserted beaches.
Vietnam Nha Trang is the hot spot with over 10 dive companies in town. Nudibrachs are very abundant here and the diving is very inexpensive.
Indonesia hard to get to the good diving, but when you do it is some of the best in the world. Indonesia sets between the Pacific and the Indian oceans at the equator, arguably the richest biosphere in the world, always in the top ten lists.
Philippines a little hard to get to but now you can get Air Asia and Tiger Air flights from Bangkok and Singapore into Angeles City and by taxi or bus get to the ferry that will take you to Puerto Galera which has it all wrecks walls and real nice reef and it is inexpensive once you get there. Make sure you are not there during the super high season, which the only thing that is super is the high hotel priest, not a good deal then.
Malaysia in places almost as good as Indonesia and not as hard to get to, and the diving can be reliably cheap. The Perhentian Islands are a great place for really cheap diving. The new King is a dive nut; This could be a very good thing for Malaysia scuba diving. Do not be surprised if he drops some cool stuff into the water to look at like helicopters, planes and big boats. I am not sure what the social protocol is when diving with a King, I would guess that swimming backwards so you do not show your butt to him could be difficult, maybe need to look that up!