On the night prior to the accident heavy rain had fallen in the area. The pilot stated that he carried out an inspection of the centre section of the strip prior to DEPARTURE and found it to be suitable. This inspection did not include the last 150 metres of the strip, in the proposed direction of take-off. The area used for the take-off roll was to one side of the centre of the strip, where the grass cover varied substantially in length and density. Standing water along the strip inhibited acceleration with the result that the nosewheel was not raised off the ground. The aircraft entered large patches of water about 150 metres before the end of the strip, causing the airspeed to decay from 65 to 40 knots. The pilot elected to continue the take-off and the aircraft overran the strip. It then sank into thick vegetation below the strip.