From Final Fantasy Wiki
The Water Flan is an enemy from Final Fantasy X. It is found in on Besaid Island appearing with other fiends. It is also the enemy during the tutorial for Black Magic in the game. It will cast Water at a single party member on each of its turns. As with other Flans, it has extremely high physical Defense but can be easily defeated by casting a spell of the opposing element, in this case Lightning. Additionally a piercing weapon with Lightningstrike will also defeat it very quickly. Though since they are already quite weak, you should not need to do so.
Water Flans will drop armor with Water Ward or Waterproof with possibly an empty slot. It will also drop weapons with Waterstrike and possibly Piercing.