It is believed that the water tentacle and the large waterfall that covered the wall around the Moon Pool was caused by the Full Moon. It was later revealed that the appearance of the Comet Eva is what activated the water tentacle. The Water Tentacle appeared in every full moon, when the Comet Eva was near the Earth to warn the mermaids about it.
Rikki Chadwick, Cleo Sertori and Isabella Hartley have all been attacked by the water tentacle. Cleo was the first to been attacked by the water tentacle, but wasn't taken to the Moon Pool. She was attacked in her bathroom, where the water tentacle came up the drain pipe and out the bath. Rikki was kidnapped by the water tentacle and taken to the Moon Pool, however the water tentacle didn't get the chance to start turning Rikki into water, as Bella and Cleo stopped it. Then in "Kidnapped," Bella was kidnapped by the water tentacle and taken to Mako Island. When she arrives in the Moon Pool, the water tentacle starts turning Bella into water, however Rikki and Cleo manage to save her.
The Water Tentacle is revealed to be friendly and it gives the Mermaids a message about a Comet Eva. The Comet entered into the Solar System on February 7th, it is also the same day the water tentacle activated.