I was wondering if I make Shadowmere unconscious and put an enchanted item with water walking attributed too it in her inventory, will her AI apply it, and allow her to walk on water?

Creatures can't equip apparel items beyond shields (if they have the capacity to do so).

The best you can do would be to cast a water-walking spell.

Madcat is correct.

I use the following combo on touch spell: Waterwalk x 120 seconds on touch + feather 3x120 seconds on self. My character equipes her waterwalking ring, casts the spell on her horse and races across the water. The feather component on her self only cost 1 extra magicka to add and it gives her a little 'timer' icon so she can see how long before the 120 seconds is up. If it is gettting ready to expire, she hops onto the surface of the water (hence the water walk ring) and rescasts the spell. Great fun!

http://i668.photobucket.com/albums/vv43/Acadian6/Buffy%20I/ScreenShot258.jpg

I'm going to try this.