Hmm. The blue lagoon was made in 1980. The strange thing is that she had shoulder to shorter than mid-back hair length in her 1978-1979 movies as well as in 1981. I've never seen Brooke Shields with longer than mid-back hair in any other movie, except in Alice Sweet Alice from 1976, see

These are behind the scenes clips and an interview from the time around 1980:

In a couple of these clips she has much shorter hair.

In a deleted scene from the movie, she appears to have shorter hair:

Anyway, this is how I want to remember the movie:

M