Mako Island's History
Mako Island was formed after a comet crashed into the Earth. We know that one of the shards impacted near Ireland (Eire) forming the Sea Caves of Ireland. So other shards may have landed in other locations creating moon pools wherever they impacted. It is possible that these other moon pools could have also created mermaids all over the Earth in other places and other times. Mako Island has a magical sea cave at its base leading to the Moon Pool where our characters get their mermaid powers. There is also a secondary cave leading to a waterfall outside. The waterfall is part of a stream running through a mangrove forest. Mako Island has been around long before humans walked the earth and it will still be around long after humans have gone. Around Mako there are the largest population of Mako sharks and sea turtles, some of which cannot be found anywhere else on earth.
Off-shore there are coral reef sands and a number of wrecks. On the northern side of the island there is a place called Triton's Reef, its a known as place where sharks breed. Throughout history, there have been peculiar happenings around Mako, such as a whirlpool and numerous ship wrecks and disappearances, giving Mako an eerie reputation.
Moon Pool-
The Moon Pool is a special, supernatural pool with the power to transform normal people into merpeople when the Full Moon passes over the top of the cave.
There are currently two known Moon Pools: one on Mako Island in [Our Island Town] and another in a Sea Caves of Ireland in Ireland.
These places were formed on Earth after collisions with a comet containing Moon Rocks and Crystal that generate special powers corresponding to the Moon.
History
The Moon Pool's magic is active only during a full moon. During this time, any human who jumps into the pool when the moon passes over, will be turned into a mermaid or merman, and given special individual powers. The Moon and the Pool generate different magnetism and effects depending on the moon's phases and the planetary alignments. When nearing collision with a comet, the pool's temperature will rise in preparation for it.
Mermaids with the power of fire: Your base power is just to be able to heat any source of water, or boil it. It can be used to raise a body temp as well.
As your power grows,
create, shape and manipulate fire, the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction products, flame being the visible portion of the fire. Depending on the substances alight, and any impurities outside, the color of the flame and the fire's intensity will be different.
Create/Generate/increase, absorb, shape and manipulate fire.
Burn/melt
Fire Attacks
http://powerlisting.wikia.com/wiki/Fire_Manipulation (Read this to get more ideas about possible power growth)
For Water: User can create, shape and manipulate water, inorganic compound with liquid, gas (steam, water vapour), turn water into steam. As it grows,
Control ocean currents.
Manipulate tides.
Tidal Wave Generation
Waterspout Generation
Whirlpool Generation
walk on water
http://powerlisting.wikia.com/wiki/Water_Manipulation
Mermaids with freezing power: The user can reduce the kinetic energy of atoms meaning they effectively make things colder, ranging from slightly chilly levels to absolute zero. As it grows,
Freeze anything to atomic level making it fragile.
Extinguish flames and cool hot objects.
Turn rain into snow, or humid air into fog.
Ice Breath
http://powerlisting.wikia.com/wiki/Freezing
Mermaid with Air: User can create, shape and manipulate air, common name given to the layer of atmospheric gases and various compounds (mostly oxygen and nitrogen) surrounding the planet Earth that is retained by Earth's gravity, and wind, movement of air relative to the surface of the planet. As it grows,
Create/generate/increase, shape and manipulate air and all forms of wind from tornadoes, hurricanes, vertical drafts, micro-bursts and other devastating forms of wind. Naturally can also create any less extreme air/wind movement.
Air Attacks
Air Solidification, Density Manipulation, and Pressure Manipulation via controlling air molecules.
http://powerlisting.wikia.com/wiki/Air_Manipulation
Please keep in mind that these wiki pages are just for inspiration. Keep things simple at first and don't overpower them or learn everything all at once.
They can only learn upgraded powers by being in the Moon Pool during another full moon (and they do not know this fact until it happens)