 The film seems to substitute the cap for the sea shell Diana finds, this also seems to be where they got the idea of taking "the item of power/birth right" from the mermaid as it is a major part of the fable and the way that Mr Fitzgerald is able to keep his mermaid wife with him much like Earl Barker does with Betty and Venus.The mermaids in the movie are also bound to grant one wish of anyone who asks it of them, no matter if it is literal or metaphorical.The three sisters in the film also have other magical abilities