Car keeps running out of water...any ideas?

You need to take it in and have the cooling system checked. If you can't see the fluid dripping onto the ground or onto the pengers side front floorboard (heater core) then you could be burning it which means you probably blew the head gasket. Check the oil. Does it look like chocolate milk? If so get it to the shop for a pretty pricey repair. It's gotta be going somewhere and you need to find out where ASAP before you completely destroy the engine. GOOD LUCK!!