During the hiatus between the fourth and fifth seasons, Rose McGowan dyed her hair red to portray Ann-Margret in the biopic Elvis (2005). The writers explained the reason for the sudden hair color change by saying that Paige goofed up while making a potion and it caused her hair to turn red.
Necron's entry in The Book of Shadows reads, "Necron is a skeletal being who hovers between life and death. This demon has the power to incinerate any living creature to feed on its life force. These stolen life forces sustain Necron through centuries of unlife, though he requires an unending supply of living essence to keep from decaying further. See Skeletal Beings for more information."
In previous episodes, we have seen Alyssa Milano's tattoo on her right shoulder. However, in some scenes when her character 'Phoebe' is a mermaid, we can no longer see it. This is especially apparent when she is on board of the fisherman's boat. Her wrist tattoo is visible at times though.
Paige jokes about how long it would take in a salon to change her hair red. In real life, Rose McGowan spent 12 hours in a salon to change her hair style. It took two bottles of lightener, a bottle of bleach, and a box of red hair dye to bring her naturally dark locks to the red color we see in this episode and for the rest of the season.
The trivia items below may give away important plot points.
When the sea demon, Necron, hits Phoebe with his lightning bolt, he strikes her right side. After Phoebe initially surfaces on the dock, she examines her wound again, which is on her right side. After she jumps back in the water to avoid being seen by the two fisherman, she is clutching her left side as if that's where the wound is.
When Phoebe is reporting back to her sisters about the call of the sea, she is far away. Then she swims toward them and pops up much closer. The size porpotion and ration between the sea and Phoebe is severely inaccurate. She appears much bigger than the sea around her.