The Water category is one of the six that are recognized in the US enemy chart and NP strategy guide. With the exception of the Wizard, these are all creatures that live underwater.
The thing about the Water category is that the Coral Sword and Excalibur have a greater effect on them. This of course only applies to the WSC/PS versions. They're all weak to lightning & strong against Earth except the Wizard, and they're all strong against Fire except the SeaTroll.