Some of the bigger multi-gate pens that SQ widebody use have little kiosks you can buy things from to take on board but otherwise no dice. You used to be able to buy things from 7-11 (in T1 at least) and they'd seal them for you but not anymore, very annoying!

You can take a plastic bottle and fill with water once past security, but otherwise SQ are very obliging once on board so I wouldn't worry too much