The water chip is only part of the main quest. Once you get/deliver it, the time limit is gone and you can keep playing as you want. Someone who's played the game more extensively can probably elaborate. If I'm not mistaken you can ignore the time limit all together and the vault will just die but that doesn't end the game(?) I might be wrong on that. Anyway, there's a whole wiki you can look at:
http://fallout.wikia.com/wiki/Portal:Fallout
Honestly, the time limit isn't a big deal anyway. You can beat the whole game well within it and there are in-game options for extending the deadline (paying the water traders to ship water to the vault for instance).