Mumbai harbour has a tricky tide, with the difference between high and low tide being about three meters (19 feet). When the tide changes, if often does so very quickly.
A board of inquiry will investigate the accident. It is already known that the navy has been tardy in awarding a contract for the dredging of the channel warships use to get in and out of the harbor. The Indian military has long been cursed with delays, often very long, in the awarding of procurement and service contracts. This has gotten worse lately because of increased efforts to eliminate corruption. In any event the channel was silting up and while there was still enough depth (at least 7 meters, or 21 feet, for a Kilo) that was no longer the case at low tide. The captain of the sub, and his navigation officer, may take a hit for this one.