Maybe this is a sign of industrialized-nation privilege on my part
but for God's sake. Water-borne diseases kill about 10,000 people every year in Mumbai.
If water-borne diseases are killing 10,000 residents a year, stop whatever else you're doing and fix that problem. Health care is state-run, so the savings from clinics alone should pay for this within a year, not to mention the increased economic activity both from the healthier citizens and the work itself will be a stimulus.
Don't worry about redeveloping the vacant mills: fix the water supply. Don't argue about allowable building heights in Colaba: fix the water supply. Don't debate whether the mandated "smoking is harmful" display that broadcasters have to put on the screen any time a character smokes is big enough (yes, really): fix the God damned water supply. This is India. You guys invented hydrological engineering and you have more civil engineers per capita than any other country.