There are currently 51 monsters that utilize the Water Element, making it the most prevalent element in the game at current time.
As of the 7/16/2012 patch, all Water monsters share the following qualities:
When battling a Water-based monster in the wild, your most effective fighter will be of the Magic or Plant element, as Water is weak against both and both are strong against Water. Using a electric-based monster is not advised although it is strong against water it is also weak against it.
This battle combination will force your attacks to have a 1.5x multiplier attached to them, while incoming attack damage will either be cut in half, or the opposing monster will use a Normal attack if it has one.