There is a nice little article in the Journal of Chemical Eduction [Negative pH Does Exist][1] pointing out examples of negative pHs in water, for example pH = -3.6 in mine water in California.  So -1.7 is not the lowest possible pH in water. Keep in mind that pH is defined in terms of hydrogen ion activity rather than concentration.  This is emphasized in [Paradoxes: Demonstrating That It Is Not True That pH ≡ -log\[H+\]][2].  At high acid concentation, hydrogen ion activity greatly deviates from hydrogen ion concentration.  For example, in 16 molar HCl, hydrogen ion activity is 678! 

However, ignoring activity and the true definition of pH, considering that pure water has a concentration of 55M, if all the water molecules were instead hydronium ions, -log(55) = -1.7.  Then in articles like [Who Knows the Ka Values of Water and the Hydronium Ion?][3] this value is attributed to the pKa of hydronium.

The article [New point of view on the meaning and on the values of
Ka(H3O+, H2O) and Kb(H2O, OH-) pairs in water][4] is a serious consideration of the true meaning of the pKa of hydronium, and points out that the -1.7 value for the pKa of hydronium has no justification. 

In conclusion, pH can be below -1.7 and the pK of hydronium isn't really -1.7.


  [1]: http://pubs.acs.org/doi/pdf/10.1021/ed083p1465
  [2]: http://pubs.acs.org/doi/pdf/10.1021/ed083p752
  [3]: http://pubs.acs.org/doi/pdf/10.1021/ed063p473
  [4]: http://pubs.rsc.org/en/content/articlepdf/1998/AN/A705491B