1. you don't want to do it unless you are under volume. don't want to thin the beer out.
2. run into possible contamination issues by adding unsanitized water. i know some people boil water, then freeze it, and add that to the wort to cool and top off.
Kegged: Citra IPA
Fermenting: Dusseldorf Alt
Planned: Black IPA, Munich Helles, Belgian Golden Strong, IIPA, Serrano/Habanero Cream Ale...