Equilibrium constant of water
In a strict sense, all quantities in the equilibrium constant are not molar concentrations but activities. Activity is a thermodynamic quantity which is proportional to concentration. Now by IUPAC definitions of standard states, the activity of pure liquids and solids is taken is unity (by convention and for reference purposes. Therefore in the following expressions, $\{\ce{H2O}\}$ or $\{\ce{H2O}\}^2$ are all unity, no matter what the power is (real number). Elementary textbooks tend to say a lot of wrong or oversimplified stuff.

$$K_\mathrm{eq} = \frac{\{\ce{H+}\}\{\ce{OH−}\}}{\{\ce{H2O}\}}$$

and 

$$K_\mathrm{eq} = \frac{\{\ce{H3O+}\}\{\ce{OH−}\}}{\{\ce{H2O}\}^2}$$

Here the curly bracket indicate activities. The IUPAC notation is $a$ for activity. The species of interest is written as a subscript.
A more detailed answer as to why activity is taken as unity is already provided:
Why are activities of solids and liquids taken to be unity?