I’m usually pretty good at finding these but not this one.
It was a teen romance that was probably published in the late fifties, early sixties. I believe the main character’s name was Cassie; I think there was also a boy named Rory in the story. I’m pretty sure there was some sort of betrayal perpetrated by those two involving a stolen kiss that was flavored by salt air, or something along those lines. It was probably mass market size and I think the color was yellow with a drawing of Cassie (if that was her name) on it.
Thanks very much!
Lost Classics of Teen Lit: Name That Book! 
Sounds like it might be “The Fabulous Year” by Elisabeth Ogilvie. Cass has a crush on Adam, but he goes away to college – meanwhile Rory has a crush on her even though she’s trying to set him up with her best friend. There’s a picture of Cass and Rory on the cover, and she’s wearing a yellow dress.
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