Nafova Timoppa (40) and his 35-year-old friend, whose name is not known yet, had gone down 24-feet-deep to install the well at Versova in Andheri (E).
The duo had allegedly not taken safety measures and died of suffocation, a senior official said.
Their bodies have been retrieved and sent for post-mortem, he said, adding police have registered an accidental death case as of now.
Action will be initiated against contractors-Sunil Karekar and Hasan-if they are found guilty of not taking required safety precautions, he said.