Low critical score
At the time I write this, the movie has 5.8 in iMDB, 31 in Metascore, 8% tomatometer and 54% audience score in Rotten Tomatoes...
The movie is unrealistic but beautiful, with excellent cinematography. It's a coming of age story about two children stranded together on a paradisiacal desert island. Their growing up far from civilization and their sexual awakening are the main themes. The nudity is tasteful, innocent and artistic. It's based on a British novel published in the Edwardian era, at the beginning of the 20th century.
It's not a perfect movie, and it won't be to everyone's taste, but the low critical score is a bit ridiculous. The dialogue is not sophisticated, but that is only appropriate given the circumstances of the characters. Are people bothered by the lack of action or by the nudity and sexual themes? Perhaps this is a movie better appreciated with European sensitivities?
The "critics consensus" from Rotten Tomatoes says "A piece of lovely dreck, The Blue Lagoon is a naughty fantasy that's also too chaste to be truly entertaining". So that's what people see here, soft-core porn, only too soft to be effective?