Found: Classic literature starts on a beach teen girl swimming to island with boy

I remember the 'taste' only of this book, as it was very slow paced and I couldn't get into it - but many times now, I've wanted to try to finish it again. It is NOT Albert Camus' 'The Stranger' but it has a similar monotone and 'older era in a foreign country' feel to it. It is an older book, 1930-1960 era?

It begins with a family going for a vacation and a teen girl that really doesn't want to be there. It is post-world-war-two era, I think. They arrive at the beach and there are lots of descriptions of 'antique' bathing costumes on some of the people and how scandalous newer bathing suits are etc. There are some elegant descriptions of the people she sees, their hair, clothes, and contrasts between them. The girl finally inches out into the water very slowly - I remember that was so painfully written I was thinking 'will this scene ever end'. She is approached by a foreign teen and they do not speak each other's language. She spends time with the group of teens over the next few days? and picks up some of the language. Then there is a bit where they swim out to an island. This was one of the 'classic literature' books I was recommended, so it is a very famous book in some circle but I have not been able to figure it out.

Well, just in case anyone else does look at this and tries to figure it out - I finally found it. It is 'Tender is the Night' by F.Scott Fitzgerald - the first few chapters only, so I had no idea what the rest of the book was going to be about.

Touchstone, Tender is the Night. Had me curious. I wouldn't have guessed, haven't read anything of his beyond Gatsby.