you can use molases the entire flowering period to help plump up the buds..
and what you are talking about is called flushing your plant.. its not done the last month.. its usually done the last 2 weeks or less of flowering... and yes you use just plain water.. but you can also use molases as well..
some people flush the last 2 weeks.. some the last week.. some just a few days before.. and some not at all.. you can play around and see what works the best for you..