What exactly is "biking distance" (<20mi? <10mi?) to the beach... when I think of "close to the beach" I think HCOL - however I live in California where anything and everything coastal is expensive. Coastal Texas for example, might be a bit cheaper.
I would try to move over the summer months to make transitioning easier. I know the 7th grader is attached, but assuming you find a good school in a new location, it's not impossible to start a new life with new friends. I dunno how school grade levels work at the schools in your area, but a move after 8th grade might mean he could start 9th grade (9th-12th grade is HS in my area) "fresh," like almost everyone else.
Like it was mentioned, crappy/unfriendly neighbors are everywhere. You will need to do research on the neighborhood beforehand. Things like block parties, community yard sales, etc. will usually be indicators of a tight-knit, friendly neighborhood. *Sometimes* few home sales can also be an indicator - if everyone has lived there for awhile, they probably know one another (but make sure it isn't like, every home is vacant or rented out, etc.).