How does water affect Mage Hand? I know there are rules regarding disorientation with ray spells, and use of fire under water. But if a person wanted to, for example, grab some keys from a well, would there be obstacles to overcome for using mage hand?
Looking at the details for Mage Hand (from the d20PFSRD):
There are no official rules, at least as far as I know, for the pressure of water on Mage Hand. However, it's possible to estimate the force of the water on the moving keys, if you're fantastic with your fluid dynamics. Supposing, though, that not everyone is an engineering major... My comment above:
Since Mage Hand has no mass, it's only fighting the mass of the keys. Thus, the sum of the weight of the keys and the normal force of the water can't exceed five pounds. It's very likely to just move slower.
However, there are many ways to read the rules. Mage Hand breaks if it encounters a solid object, so you could just say it fails underwater. You could also read the rules explicitly; since the object is under five pounds, you can move it fifteen feet in any direction, obviously unless it's obstructed.