Science Project help..?

I am attempting to make a miniature submarine for a science project. It's not that far fetched. I need Ideas on how to pump the water out of my "ballast" tanks (sorry if I spelt that wrong). I am either going to use milk jugs (reinforced with something of course) or or huge pvc piping with a cap. But i need Ideas on how to make the water go out when I want to go lower in water.