I would like to use a photocell to turn on a pump for a water feature during the day, but be off at night. This is the opposite of the traditional use of a photocell, to allow a light to be on from dusk to dawn. Is there a way to rig a photocell so that it will work during the day and be off at night? If not, is it possible to buy one that would be on during the day?

You can make a circuit that turns it on when there is light and turns it off when it gets dark the same way street lights work. (just opposite hours of operation) The circuit your looking for is called "normally closed" from the photocell. When enough light is generated to activate the photocell then it "closes" the circuit. When light diminishs it "opens" the circuit cutting off electricity. You will also need a relay in the system to absord the power of the pump and prevent overheating of the circuit.

Simpler and cheaper to just buy a timer.