The amateur-built airplane was substantially damaged during a forced landing during landing approach. The pilot reported that as he circled the airport, the engine speed went to 6,000 RPM. He stated that he pulled the power back, lowered the landing gear and increased the propeller pitch. He said that the airplane began to stall and he could not maintain altitude. He executed a forced landing to a wetland. The right wing struck the water and broke off spinning the airplane around. Subsequent examination of the controllable pitch propeller revealed that a jack screw pin had sheared allowing the prop to go to a fine pitch setting. One of the blade pitch stops also broke resulting in the blade moving into a reverse pitch position.