When I moved south in the late 80s a lot of people were talking about Beach Music. I had no idea what they were talking about, I thought it was old Beach Boys Records.
It is not a separate idiom but culls from many and any that feature easy music to party to in the summer, usually in the middle aged and up demographic. Here’s one radio station that will give you a feel for it.
https://beachwavesradio.com/