For KVPY water bottles were allowed. Security at exam halls (I.e. methods to prevent cheating methods like CCTV are there, however checking for bottles etc didn’t take place) are present. In fact we were even allowed to take our pencil pouches inside the examination hall. As far as rough sheets are concerned... it’s of A3 size ruled. Paper quality is not as classmate but it is manageable. A bit lower grade than board answer papers. Rough sheets are unlimited.
Pls note digital watches are not allowed. Although you wouldn’t need it coz there is a timer in built to the software (at least there was one for KVPY)