What do deer lick?

They don't lick up water; they're not cats or dogs. Deer, essentially suck up water, using a vacuum effect with their mouth and tongue to bring water into their mouths before swallowing.

It's just like you would drink from a wide bowl of water: instead of tipping it towards you and have the risk of having all that water splash down your front, you just leave it on the ground (or table or counter-top) and use your lips and tongue to suck it up into your mouth.