Key Largo, is a beach, where Blaire's father, Mr. Harris. Brings Blaire fishing every year. He brings Blaire there in hope of seeing Terra again. But never finds her, people are now thinking that he will still go there, to look for both Blaire and Terra.
There was also 1 or 2 episodes filmed there (really at a beach near Amy's house) in season 4, while the girls were trying to get back the evidence of real mermaids, that Tess ended up sending to the marine biology center .