During the takeoff roll from San Juan-Luis Munoz Marin International Airport, there was a fluctuation of Brake Mean Engine Pressure (BMEP-a measurement of engine output), on the no. 2 engine. At this point the pilot elected to abort the takeoff. No abnormalities were noted during an engine run-up near the hangar; the airplane was taxied to the departure runway where another engine run-up was performed with no abnormalities noted. After take off (at 13:15 AST), as the airplane climbed through 200 feet MSL, the no. 2 engine backfired, the BMEP indicator fluctuated and dropped showing about 150 BMEP difference with respect to engine no.1, oil temperature started to rise rapidly, oil pressure dropped and the airplane started to vibrate. no. 2 engine power was reduced to “dry” power, upon which a second bang or backfire was heard from the no. 2 engine. The no. 2 propeller was then feathered by the first officer. The airplane was leveled off at 400 feet MSL where, in an attempt to return the airplane to the airport, altitude could not be maintained and the airplane was force landed in a salt water lagoon some 2 miles short of runway 28. The airplane came to rest with the left wing separated in approx. 8 feet of water. Due to hampered recovery efforts brought about by recently passed Hurricane Georges, the airplane was not recovered from the salt water lagoon for 70 days. PROBABLE CAUSE: “The loss of power in the No. 2 engine for undetermined reasons, and the inability of the pilot to establish a climb and/or maintain altitude. A factor was the incompletely feathered No. 2 propeller.”