Link will mainly be challenged by the Water Temple's excruciating puzzles, but he'll encounter aggressive enemies along the way as well. It has been many years since the Temple's halls have been disturbed, and the creatures which call them home do not passively retreat when an outsider disturbs their dwellings.
Blue Tektite
Undoubtedly related to the Red Tektites who live on Death Mountain, Blue Tektites prefer to live near water. They can skim across the water's surface with efficient speed, or hop across dry land to pursue their prey.
Stinger
Stingers are well-adapted to life in the Water Temple, for they can breathe underwater just as easily as they can breathe while flying through the air. Solitary Stingers are not terribly threatening, which is why they always attack in packs.
Spike
Spikes scurry along the bottom of the Water Temple's waterways, looking for intruders in their territory to attack. While resting they resemble simple rocks, but when agitated they sprout metallic spikes ready to skewer anything in their path.
Shell Blade
Shell Blades are aggressive crustaceans which are surprisingly mobile for their simple anatomy. Razor-sharp teeth growing on the outside of their shells suggest a carnivorous diet, so beware of their steel-trap mouths.
Keese
Keese have invaded nearly every gloomy location in Hyrule, and the Water Temple is no exception. Stingers are more suited for an aquatic lifestyle, but the Temple's vaulted ceilings can provide a comfortable hiding place for the flying furballs.