Private pilot of a Piper PA-28R-180 departed runway 15 at West Houston Airport (IWS), Houston, Texas. Pilot stated he had departed runway 15 when engine quit at approximately 600 feet AGL. Pilot executed a forced landing approximately 2 miles off the departure end of runway. Pilot stated he failed to deactivate the automatic gear lowering feature of the aircraft which resulted in a forced landing into an unimproved area with all three landing gears down and locked. Aircraft sustained substantial damage. Nose gear sheared and both main gears were forced upward into the wing structures. The pilot and one passenger onboard the aircraft were uninjured. Pilot stated he took on 27 gallons of low lead aviation fuel at West Houston Airport and had drained fuel samples from wing tank sumps and gasolator and found no water present. However, investigation of the aircraft subsequent to the accident revealed some water present in the gasolator. Visual meteorological conditions prevailed at the time.