The student pilot/owner reported that he was flying eastbound with a passenger and was following two dirt roads, a set of power lines, and a canal that were all oriented east-west and parallel to one another. As the airplane flew along the canal, a "downdraft" or "rotor" caused a sudden loss of altitude, which placed the airplane directly in the flightpath of an intersecting (north/south) set of power lines that crossed the canal. The pilot attempted to fly under the power lines to avoid them but struck a tree on the canal's bank. The airplane then impacted the water and submerged, which resulted in substantial damage to the airframe. Witnesses reported observing the airplane flying just above the canal, below the height of the parallel lines, when it descended further, struck the tree, then the water. The student pilot/owner reported no mechanical malfunctions or failures with the airplane that would have precluded normal operation.