Can you help me find out name of this movie?

Okay, when I was younger I used to watch a movie at my grandmother's
house a lot. I'm not exactly sure what time period the movie was set
in, but I believe it was the 50's. It was about a blonde girl who
didn't live to far from the beach.

Here are a few scenes that I can remember:

- The girl goes one day and there are a few surfer guys and she asks
if she can join them. They tell her she can join them if she brings
them seaweed from the ocean floor. She goes down a few times and
brings them seaweed but ends up getting caught and almost drowns, and
one of them saves her.

- The girl tries to hang out with the guys a lot because she likes one
of them, and the guy is mad that she's trying to change herself.

- There's a party at the beach one night and she leaves with some old
pervert who tries to have his way with her but the guy saves her.

- The girl's parents are friends with the guys parents (they both were
unaware of this) and so they end up getting together at the very end
of the movie.

I'm so sorry that this is so vague. :(
But if you can help me I'd be so thankful!!

You don't have to know exactly what movie it is, if it sounds familiar
and you know a movie like this, please give me a name and I'll look it
up!! Thanks so much! :)