you mentioned SEA but the area is VERY broad, Indonesia for instance is more than 14.000 islands...being more specific would help to answer...
However, here are some hot spots :
- Eastern peninsular malaysia (Perhentians, tenggol) october may still be Ok, closed from november reopening usually from Chinese new year (feb). Forget west coast, there's only one spot on Pulau Payar which is not worth going.
- Borneo (Sipadan, Mabul, Lankayan, Layang-Layang, Derawan) : all year round but better fro March. said as some of the best dive spots around but can be very expensive.
- Indonesia : plenty of diving !!!!
Sumatra, Pulau Weh inexpensive diving and lodging all year round.
Bali from march to october, Candi Dasa, Tulamben and Amed spots are definitely worth.
Sulawesi : Bunaken, Lembeh Strait, Bangka, Makassar all year round but rough seas from November to March. inexpensive too if you choose the right dive centre and accom. Sulawesi has may be the greatest dive potential of all the indonesian islands : you will always find somewhere to dive, but it can be hard to get to it (Tukang besi, Selayar, Togeans, ...).
There are also hundreds of places that provide world class diving in Indonesia (Flores, Kupang, Banda islands, Irian Jaya,...).
-Philippines : Visayas diving is year round specially Moalboal. Palawan and Puerto galera better from January to May. January to April is whale shark season in Cebu and Leyte. Diving is some of the most inexpensive in the world.
Forget Vietnam for diving. I'm quite critical about Thailand which is really a second option to other stunning spots in the SEA area.
