H2O: Substanciakinesis

Substanciakinesis is the power to turn water and all things that contain water into a substance similar to crystal or glass.

With this power, merpeople can harden the water in their favor and render it indestructible. It was also shown that if Bella wants, the water will return to its normal state after a few seconds, but if she doesn't want it to, the water will forever be rigid

Speculations
There are many things about this power that are still unknown. It can be guessed that Mako Island and the Sea Caves of Ireland have different water-affiliated powers and abilities to be given.

Another theory is that the Moon Pool gives water-based powers of the four nature elements because Cleo has aerokinesis (air), Emma has cryokinesis (water), and Rikki has pyrokinesis (fire), so when Bella's power upgrades she might get geokinesis (earth) - the power to control the earth. However, because the series has ended, it is unknown if she ever enhanced her power.