Water is likely to have been sourced in original minerasls and objects bombarding the forming planet. The minerals forming in space would have picked up water molecules before slamming into the early earth. The interesting thing is to understand how it is that a particular zone of solar system formations favors water over another region of the forming solar system. Since all the stuff of planets and stars now existing came from the furnaces of earlier stars, why did a general region of higher concentrations of water happen in our solar system?
Your initial sentence seems to slightly contradict the last sentence.