You're crazy for liking Waterworld, and I'm crazy for liking David Lynch's Dune, so I guess that makes us even.
The thing I liked best about Waterworld was Costner's character - the idea that a human could adapt to life in water and grow gills. It just seemed a rather fast adaptation, in evolutionary terms. Shouldn't such an alteration to the human body take thousands of years? Mind you, I don't remember how far in the future the story takes place.
Just thought of another film that I enjoyed for its time: The Road Warrior, the 2nd entry in the Mad Max trilogy. I don't think I've ever seen the original Mad Max in its entirety, but I saw enough that I can say it didn't captivate me all that much. The Road Warrior was the one that really caught my imagination; for a brief time, I regarded it as one of the best action movies ever - and maybe it still is for many...but over the years, the film has lost a lot of lustre in my eyes. However, I think it's still the best of the trilogy. Mad Max Beyond Thunderdome had its moments; indeed, at its best I think it is the most poetic and poignant film of the group. More so than the others, Thunderdome conveyed a sense of sadness for the destruction of civilization, the loss of culture and language, as personified in the band of lost kids that saw Mad Max as their savior. The problem for me was that the film really dragged in places - I'm sure I nearly dozed off a couple of times. Whereas The Road Warrior was more focused.