They certainly look freshwater, The leaves in the second and last photo look like they're covered in epiphytic algae which makes identification trick6y but try this
I also agree with Dr, Abhijit Mitra, it doesn't seem to be sea grasses. Check it with any species of Chara or Nitella (2-4). For the first picture, pl check Scirpus sp.
I guess, the second and the fifth are images of Ceratophyllum (C. demersum?). The third and the fourth are charophytes - Lamprothamnium (L. succinctum?) or Chara (totally ecorticate species). Please check guide for Indian charophytes http://krishikosh.egranth.ac.in/HD/common/split_document.jsp?doc=IISR-1117.pdf1/2061654/2/
If you collected several specimens of charophytes, it seems to be quite easy to identify them with the help of this book. I could help you with identification but more close-up photos or specimen study are required for this.
Ceratophyllum sp. on the 2nd and 5th pictures. Probably a Chara on the 3rd and 4th (acorticate one). Follow the advices of Roman for the identification.
Yes as identified by Boissezon, third and fourth pictures are Chara sp, but I dont think its corticated....looks like an ecorticate member to me . Second and fifth ones are Ceratophyllum sp..