The teacher, Martin Amunue (49), based at Onayena Primary School - about 27 kilometres east of Ondangwa - was found dead in his car on Saturday.
Oshikoto police deputy Commissioner Wellem Peter said deducing from the tyre marks, they suspect Amunue approached the dam slowly and then drove at high speed into the deep end.
Peter also said the vehicle's doors and the windows were shut when the vehicle was pulled out of the water.
He also said the vehicle was not damaged and there was no suicide note.
Villagers told The Namibian that Amunue was seen at the village cuca shops with friends but he left early.
Unconfirmed reports say that people who were walking near the dam saw Amunue driving into the water and alerted other villages who called the police but because it was late, police only arrived on Saturday.
Villagers including Amunue's wife and children gathered at the dam where a small part of the pick up was visible. The police could not pull the vehicle out because their divers were attending a workshop at Walvis Bay.
A local businessman assisted the police with a hoist to pull out the vehicle with the body.