The Mermaid Comb of Merroway Cove is an enchanted hair comb with the immense hydrokinetic ability to control all of the seventeen seas and oceans.
The comb was given to Oona by her mother, much to Cora's disappointment. She was told it possessed great magical abilities but would not tell her until she was old enough.
It was said that Queen Emmaline and Cora only knew about the comb's magic but it turned out so did Cedric. He came along with the royal family aboard the Floating Palace to get his hands on it and use it to take over the Kingdom of Enchancia. Cedric and Wormwood, as sea creatures, tried to take the comb by force but failed because it had a protection charm placed on it. After capturing Oona, Cedric used his magic to try and break the charm and remove the comb from her person. He successfully broke the charm and removed the comb from Oona's person but Sofia intercepts it and tosses it to Cora, who used its hydrokinetic powers to defeat Cedric.
In "Cool Hand Fluke", it is revealed that Cora has shown Oona how to use the powers of the comb and has led to Oona using it herself when she made water spout around the Mer-Tones while they played music for Sofia's family.
In "Return to Merroway Cove", Oona is sick of being told by her mother about being too young to handle the powers of hte comb, so she sets out to prove her wrong. She creates a sea twister that soon gets out of her control. She ignores Sofia and Sven's requests to get her mother’s help, fearful that she’ll be furious at her mistake. Sofia gets sucked into the twister where she is saved by Nerissa. Eventually, Oona notices the twister heading stright towards the palace, and reluctantly agrees to get her mom's assitance.
Oona arrives at the palace, but her mother isn't there, only Cora. Cora suggests to use their mother's trident ot stop the twister. Suddenly, Queen Emmaline arrives and stops the rampaging sea twister with the trident. As it turns out, Sven got her to help. Emmaline scolds her daughters for using the trident and the comb without permission and confines them to the Palace. However, because they apologized and told the truth in the end, she allows them to take part in the show after all and announces the start of the confinement for tomorrow.