Sofia the First: The Floating Palace(2013)

Overview:

After Sofia helps a little mermaid named Oona, Sofia's amulet grants her the power to transform into a mermaid and visit her new friend's underwater kingdom. But when Cedric (disguised as a seamonster) plots to steal Oona's enchanted Mermaid Comb, danger surfaces for both worlds. With a little help from special guest, Princess Ariel, Sofia embarks on an adventure to rescue Oona and prevent a typhoon before it's too late.