I would say IITs like Delhi, Bombay, Kharagpur, Kanpur, Roorkee etc would be the best choice comparatively to all institutes to pursue M.Tech in Hydrology so it would be better if you try to get admission there but yes, unfortunately not all IITs offer M.Tech in Hydrology except IIT Roorkee. Also, there is nothing declared for GATE 2018 cut off but in previous year the cut off had gone to 48.76 for general category.
Apart from this, if you complete M.Tech in Hydrology from IIT Roorkee then the scope is decent as you can enter into PhD in good foreign universities as well as in IITs and IISc and pursue your career in research and academics but the only condition is you should be research oriented and as per my view the industrial jobs in this specialization is little less so I would suggest you to try for teaching career but before you make any decision kindly consider your interest and potential.
Hope this helps!!!