Although both are important, I would disagree with the comments from this blogger at http://www.AskAquaPro.com . Both oil and water are non-renewable resources. However, I believe that de-salination technologies will continue to improve in recovering fresh water from sea water. There are also rain-making technologies in the works. Finally, when the use of oil and its products is greatly reduced due to Peak Oil, global warming may slow and even reverse. This would reduced the threat of Peak Water.
The primary reason, however, that I believe Peak Oil deserves more attention is as follows. The technologies I mention above are currently dependent upon oil for fueling them and manufacturing them. Without oil, the technologies for accessing more water will not be able to achieve their greatest potential. Hence, Peak Oil will be one of the great causes of Peak Water.