At the end of a 180 deg turn the pilot attempted to level out but there was no cyclic response. The helicopter continued in the turn and the application of back cyclic could not prevent the nose dropping. The helicopter struck the bank of a creek shearing off the tail boom, main rotor and gearbox and came to rest in the water. The pilot escaped unhurt and unaided from the partially submerged cockpit. Investigation revealed that a bolt which secured a foot of the cyclic control wobble plate pylon to the transmission gearbox had become detached. Subsequent movement of the pylon deprived the pilot of effective cyclic control. The reason for the loss of the bolt could not be established.