If you can't see any sign of a leak, have a look round the header tank filler cap and overflow pipe for signs of coolant residue, if present then pressure in the cooling system is pushing it out. If you only loose water when the aircon is on this could be cased by the engine overheating because of the heat produced by the condenser which is in front of the radiator, if this is the case you would notice a rise in temperature. If this isn't happening its likely that the head gasket is leaking.