That's it exactly. The critics simply decided it was time to trash Kevin Costner, because he had become too famous and popular, and they went after him with a vengeance...and people in general simply jumped on the bandwagon.

Waterworld had a number of good points. It was a pretty decent action and sci-fi film, and it did not deserve the amount of abuse it got from the critics.

After that the pattern was established. If Kevin Costner did any new movie, specially a big movie...everbody was just waiting to pan it unmercifully, their mouths watering at the prospect.