I'd have to recommend Pulau Weh, off the north tip of Sumatra. I'm going for the third time in July. The diving is amazing, accomodation and diving is cheap. A boat dive is $16 (if you do more than 6 dives) incl all equipment. The house reef is not bad at all.
There are two beaches, Iboih and Gapang. I've stayed and dived at the latter, the divecenter is called Lumba Lumba, run by a very friendly dutch couple (web page www.pulauweh.com - don't be fooled by the crappy design of the web page
). I suggest you email them and ask them about whatever you want to know. They are also more than happy to help you with flight schedules and such.
You can fly from Singapore to Medan and then on to Banda Aceh ($40), and on tuesdays and thursdays there is a SMAC flight to Pulau Weh ($7). So it's only a couple of hours from Singapore.
Although there has been some fighting between the separist movement and the indonesian army in Aceh, Pulau Weh has always been safe.
It's really a nice place, and the diving is spectacular. I'd recommend it to anyone.
Paul, Sweden