Keshav if you are able to get the option of doing M.Tech in structural engineering from the top NITs like NIT Trichy, NIT Surat, NIT Warangal then you should definitely go for it over doing M.Tech in Water Resources Engineering in IIT Guwahati because the on-campus perspective is not so good in IIT Guwahati. If you look at previous year records hardly anytime full M.Tech batch gets placed. Even you won't find an amazing placement in top NITs but still, in comparison, they have a little better placement plus with your structural engineering branch, lots of government job opportunities will open for you. You will have more scope in off-campus placements. And in the ned also look for your interest, if you are more interested in the water resource engineering then you should listen to your interest because you will be able to perform better when you will like the course.

Feel free to ask if you have any more queries. All the best!!