Suggestions for preventing water from building on a shower window that is below shoulder level?
If you've got water collecting there, I think your best bet would be to prevent it from getting there in the first place.  A curtain over the window well would probably be best as that would allow you access to the window if you need it for ventilation.  

I also thought about the thermal or decorative window films, cutting a piece to cover the bottom of the window and extending down over the frame and sill onto the bathroom wall.  Don't know how well it would work or if it would look OK, though.