Water Engineering

The college does not specify the scholarship criteria for the courses. It might be the score in the entrance or your performance in the interview or a slew of other signaling things in your CV. It would be prudent to check it with the college once. You can get the number for the TERI SAS website.

Yes, it is good.
In NIT Silchar, M.Tech in water resource engineering is a Post Graduate programme which has wide scope in water management, resources, etc.

Please refer to the link provided below for a list of colleges which offer M.Tech agriculture and related courses. You can download their brochure and check if distance courses are offered by the institute:
https://www.shiksha.com/engineering/colleges/m-tech-agriculture-farm-engineering-colleges-india

I am the one who choose Masters in civil engineering with the specialization of water engineering, so in my view, water engineering is the most important subject of civil engineering after structural engineering, everyday world is busy to save water for future , so the value of a water engineers increase every day, hence go for it.