Adapted from a children’s book by Alice Hoffman, this first feature by Elizabeth Allen can be read as an allegory about teenage girls navigating puberty. Two best friends (Emma Roberts and Joanna Jojo Levesque), both smitten with a hunky local lifeguard (Jake McDorman) in their Florida hometown, discover a mermaid named Aquamarine (Sara Paxton) who’s been washed ashore during a storm. She’s trying to escape from an arranged marriage and has three days to prove that true love exists, so the friends try to nurture a romance between her and the lifeguard. The story has its hokey moments (There’s something very fishy about that girl), but the sincerity and focus of the storytelling compensate. PG, 109 min. (JR)