If anything brings it down, it's Kevin Costner's performance. The brooding, gloomy hero shtick works, but he's still far too serious for a movie like this.
If he'd had the good sense to be a little more tongue-in-cheek, it would have helped a LOT.
Still, it's not a bad movie by any means, and I really think all the negativity toward it is unnecessary. I want the Costner bashers to sit down and watch 'Rebirth of the Mothra III' on Sci-Fi Channel some time. Then they'll know that they have seen the worst movie ever made. 'Waterworld' is an escapist fantasy/action picture, not unlike the James Bond and Indiana Jones pictures.