Beastiary : my own version

Naiads

Naiads, also known as Water Nymphs, Neptunes, Nixies, children of the Sea or as mermaids and mermen, are a water-dwelling species of Seelie, they are normally more native to the Seelie homeland of Avalon but some colonies have become native to the Human world. When they shift they're eyes glow an aqua green, they gain slightly scaly patterned skin, silvery-blue tinted webbed hands and a pair of small curved gills on each side of their neck which can be used to breath underwater. They can also gain a fishtail like mermaids and mermen in mythology however this is only when they are submerged in water. The colouring of Naiad's features can however differ slightly from one to the other but the majority of the species have the ones previously stated.

Behaviour:
Naiads are usually very energetic, reckless and enthusiastic. They can also appear to act strange and out-spoken to humans due to almost all of them being raised in their homeland of Atlantis or in the sea colonies on earth. They however must travel onto land to conceive and birth their young as they can't procreate when transformed in water.

Powers and abilities:
Naiad's like most supernatural creatures have there own unique traits which in this case are a selection of special water-based and none-water based powers.

Longevity/Semi-immortality: When in water a Naiad will not age once reached maturity granting them an enteral lifespan however if a Naiad should chose to walk on land they will no long be preserved by the water around them and will start to age however they still do age slower than humans by a substantial rate.
Hydrokinesis: They have the ability to control, alter and shape existing water or water they have summoned, they can use this for both offensive and defensive means such as shaping water into balls or streams that can be fired at enemies or they can shape water into walls or bubbles for protection. They can also learn to freeze water into solid ice or evaporate it into steam.
Water summoning: Naiad's can summon water seemingly out of thin air. They do this by condensing the moisture in the air into water however this is normally not there first option as the amount summoned is limited and exhausts them. However the more skilled they get at using their abilities the less this becomes a restriction.
Water-healing: By using water as a magical conduit they can heal themselves or others of minor or serious wounds as well as some diseases and poisons. When they do this the water their controlling will illuminate the same colour as their eyes.
Fish physiology: When shifted they can breath under water and withstand the pressure of being under deepwater, they can also swim at extreme speeds similar to that of a fish. They can also see clearly underwater without there vision being effected by salt or water-based bacteria.
Water empowerment: By absorbing water they gain increased strength, stamina and magical ability however this is only temporary.
Toxin Immunity: Naiad's are immune to the majority of Earth based toxins and poisons.
Aquatic telepathy: They can telepathic communicate and control aquatic-life, such as fish, turtles, sharks, Dolphins..etc
Naiadian magic: Naiads have access to their people's ancient magics capable of placing magical enchantments as well as summoning semi-sentient beings out of water to work or fight along side the naiad however these beings have a short half-life. A Naiad's capability in this water-based magic is completely determined by the user as it is something learned and practiced rather than an inherent ability.

Weaknesses:
Naiad's like all supernaturals have there own weaknesses to way out there strengths.

Physical trauma: Despite there healing abilities they can still be killed by excessive physical trauma and mortal weapons.
Morality: If a Naiad choses to live a life on land they will age and eventually die the same as humans however this process is slowed down greatly.
Dehydration: They need to consume or absorb way more fluids than humans therefore can become dehydrated quicker than humans.
Fire and high temperatures: Due to there dependence on water, fire and high temperatures can cause them to become nauseous, exhausted and can diminish the effects and capacity of there powers. If subjected to enough heat for too long they can become incapacitated and even die.
Iron: Iron burns more pure blooded Seelie to the touch but is described as more of a frozen feeling rather than burn hence the term "cold iron". Only pure iron affects them as steel and other iron alloys don't bother them.
Rowan wood: Seelie magic is weakened and doesn't work on objects made of rowan wood.