Minecraft wiki entry for Water_gate, associated text: Cactus (Not the best, needs sand under it, but is good for hurting invaders or mobs)
Sugar Cane (Requires dirt or sand underneath, and is a good blocker. Also, able to see through as a window)
Cobblestone (Doesn't work well, but if you are low on materials it works.)
Glass (Allows items to go through it, which is beneficial, but requires lots of smelting and sand.)