Let's see. My choice destinations from boarding flight to getting wet in under 7 hours = Bali (great shore or short boat dives along East Coast (Padang Bai, Ahmed, Tulamben etc)) Not much sun now since rainy season is starting.
under 9hrs= Anilao PI... VERY traffic dependent but good mix of muck and reef.
Manado= night dives around Manado are actually quite good but all muck. Boat dives but short rides.
Lembeh= farther than Manado, but fantastic muck dives. Depending on resort you can have awesome dive sites as housereef.
Medium hot:
P. Tenggol= best dive area in Peninsular Malaysia. No direct flight from SIN (that I know of...but I haven't been in 3 years at least)
Just to get away and wet choice:
Tioman= flight from SIN
Perhentian= Same as Tenggol but farther north.
With Phuket/Samui, even if you arrive by noon, the only way to dive is to hire a speed boat to take you out to the dive sites as the day boats leave in the morning. If you are very eco-unfriendly and make your carbon foot print and credit card strip felt for sure
, charter a speed boat to the Hin Daeng/Muang or Similans. 
Max, it's monsoon but the dive ops still open. It's just rainy but many sites still diveable... esp in Tenggol. Viz won't be great though.