Well, sure the US could decide to nullify the treaty. This would almost certainly be abysmal for Mexico/US relations. Suddenly when the US want's Mexico's help to do something, Mexico will be much less willing. Not to mention the people living along the border that need that water to survive. What will they do when they start to get really, really thirsty? I'm betting it won't be sitting on their hands waiting patiently for the US to turn the water on again.

If tensions get really bad, sure the US armed forces could probably take Mexico in a fight, but then what? What good would that do the US?

It would be a pretty idiotic thing to do, but sure, it's possible.