The Marina Bay Sands has a water show that’s put on every night for free at the Promenade, called Event Plaza, in front of the shops. On school nights it is on at 8pm and 9.30pm, but on Friday and Saturday nights there is another show at 11pm.
The water show is put to some lights and music and lasts 15 minutes. There is also a light show with lasers from the top of the Sands Hotel at the same time. Its for this reason I would recommend seeing in twice in one night if you can.