If you watched the original
Blue Lagoon (1980), then the only reason you can watch this sequel is to see a young Milla Jovovich. Which, by the way, doesn't look like itself on the poster. On the poster she is just old, and in the film is really a young girl, more round-faced and not at all like her classic adult “vampire” image. There is no other reason to watch the movie.
This formally sequel (that is, the plot continuation) is largely a remake of the first film. The story has changed, but it feels like it’s secondary. The first film looked alive, and the characters were naive in a good way, there the growing up of the characters is well shown. It's all theatrical.