Vis is pretty nasty around Singapore so its not ideal. Singapore is only half a day's bus & boat ride to Tioman Island in Malaysia, which has good diving - macro, schooling fish, wrecks, hard & soft coral sites, swim throughs... The vis drops late in the summer but it is still decent. There used to be a direct ferry but I don't know if that is running any more. You could also charter a long distance taxi in Malaysia if you don't fancy the bus.
If time allows, Singapore is a great hub to get to Manado or Bali through the budget airlines.