Help! Whats the title of this book!?

I remember reading this book with my class either in late elementary or in middle school maybe. 

It took place in a coastal town... I think the title had something to do with beaches, oceans, or seashells? Its a coming of age kind of story where the main character for some reason ends up living with her cousin. She and her cousin hate each other or don't get along at least. The main character is brunette if I remember correctly more of a reserved character. The cousin, I think her name is Faye or Fannie, is blonde more promiscuous and is dating a much older guy in his 20s. The older guy is a sailor and doesn't know the blonde girl is around 15 yrs old since she lied about her age. At some point the main character reveals the blonde girl's actual age to the older guy and he freaks out and slaps her and dumps her right away. 

It kills me that I can't remember the name of this book, any help is appreciated!!

Update:

For clarification this had to be a book from the 90s - early 2000s. Maybe earlier than the 90s but def isn't anything recent.

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