Why evacuate wing at the front side after water landing?
When I worked as a flight attendant back in the 80's we were told that water landings frequently resulted in very damaged trailing edges to the wings.  Flaps and spoilers will most likely have been deployed prior to landing and the velocity of the water impacting these extended surfaces would tear them up badly, along with the hinges and fairings that support them.  To avoid serious injury to passengers from ragged sheet metal, we were to direct them off the leading edge of the wing.  

This wouldn't be a problem with an evacuation on land so in that case we would direct them off the trailing edge.  The upper wing surface makes a good slide and a more natural and controlled drop to the ground.  

Edit:  I forgot to mention that additionally, water landings mean passengers would be wearing life vests and life rafts would be deployed.  Jagged sheet metal would make short work of these inflated devices as well.