I did some research and this is what I found:
"Endermen are damaged by fire, lava, water and attacks. Emptying a bucket of water on the ground will damage the Enderman without it becoming hostile, though it will teleport away. All types of swords can be used to kill an enderman. Cacti will also damage Endermen, so a wall of cacti also serves as a passive defense against Endermen.
Endermen can't swim. If the player is in a large body of water, he will teleport toward the player, take damage from the water, then teleport away. They could take enough damage from the water to die."