Window AC units are often installed in homes with little thought given to how they will be vented. Improper ventilation can lead to condensation and water leaking from the unit. Make sure the unit is properly vented. The vent should be clear of any obstructions and the opening should be the correct size. Also check the unit's drainage system. The drainage hose should be clear of any obstructions and the end should be placed in a drain or bucket. If you have tried all of these things and the unit is still leaking water, you may need to call a technician to investigate the problem.