Adding water will work to dilute the brine, but please remember that it will also dilute the Calcium concentration. If too little of that, then the Feta will tend to start "melting" into a gooey substance!
If you go the dillution route, add some CaCl to the brine. Of course, it would be best if you still have some of the whey from the make to use to make new brine, but that's probably unrealistic.
For my taste, I am finding that a 7-8% brine solution works.