A Boeing 737-247, operated by Air Tanzania, suffered substantial damage in a runway excursion accident after landing at Mwanza Airport (MWZ), Tanzania. The aircraft had departed from Dar es Salaam. The en route part of the flight was uneventful. At 07:22 hours the flight contacted the Mwanza Tower controller and requested information about the location of the CB cloud which had earlier been mentioned by another aircraft in the area. Mwanza Tower informed the flight that there was a CB lying north east of the airport. The relevant METAR weather information was also transmitted to the flight. The pilot subsequently requested to use runway 30 and was cleared to do so. By that time the wind was 030§/18 kt and the visibility was 10 km. The aircraft proceeded to make a visual approach to the airfield and later reported field in sight. The captain was flying the aircraft. He testified that the approach to runway 30 and touchdown were all normal. He also claimed that after reverse thrust was selected, the aircraft entered a puddle of water that adversely affected its performance and controllability and caused it to veer off the runway. After covering 305 metres on grass, the aircraft came to rest on grass off the left edge of runway 30 some 1450 metres from the threshold. About 5 metres of the right wing was protruding into the runway and the nose section was resting on the tarmac surface of a service road.