Is there concern about water penetration in Kolkata? I've always been entertained, if that's the right word, that this was India's first subway system and then it just stagnated for years and years...
Kolkata is on an average about 17' over MSL and its water table is 15' or so from the surface. So of course everything underground has to be designed assuming it is under water. Yet the Calcutta Electric Supply Corporation dug the first under water tunnel in Asia under the Hooghly River (33.5m below ground level) to carry power cables back in 1931. So Kolkata is not new to doing stuff underground. What slows things down is idiotic politics that the locals excel at, usually to their own detriment. but that is a subject for another forum and another day.
The under river portion of the dual tunnel with escape cross connects and sectionalization flood gates is 30m from roof to ground level. Howrah Metro Station is also 30m below ground level (track level is of course another 5+ meters further down.
Spring tides which come up the river in the form of impressive tidal bores usually flood many areas close to the riverfront, and have done so for a hundred years. People are used to it. They know when it will happen. They just step out of its way when it comes and then continue with life as it recedes. There are many areas in Kolkata where in order to enter and underground walkway or Metro station you have to first climb up stairs 4'-6' to make sure that water from water logging due to various reasons does not get into the underground structures.
Building tubes is expensive. That is why most of Kolkata's Metro Lines are elevated 30'-60' above street level. They are underground only in the city center and that too, Line 1 was cut and cover. Only Line 2 is deep tube. Line 3 I believe is going to be mostly shallow tube since it is mostly under the Maidan into Esplanade. A short deep tube portion at the south end of the tunnel portion will be used to go under the now moribund old main channel of the river.
Anyway, very soon now, a significant proportion of the million people that use Howrah Station each day will start using this remarkable new Metro Line.
Incidentally, Kolkata Metro is a Division of Indian Railways, and the only one which is run by Indian Railways. All other Metros in India are run by self-standing separate organizations.