I don't get why everyone hated the movie. I thought it was cool.
I haven't played the game, but on the occasions I've seen it, the back glass has not pulled me in.
I think the dislike of the movie comes from several factors. Costner didn't really turn in all that interesting of a performance. Competent, sure, but nothing to write home about.
The pirates in the old tanker were more than a little odd (where'd the jet skis come from again? And how did they keep them running for several generations?).
The girl with the ink on her back: I'm supposed to believe that the parents, lacking any other suitable storage method, inked the information into their child's back? That's gotta hurt, and take a long time, and the kid would have been pretty small. I sure as hell wouldn't have wanted to hear the kid scream that length of time.
The big WTF moment in the movie for me was the 'diving bell' scene where Costner takes someone down to look at the sunken city. You're telling me that a guy who has gills takes up the the space and weight of that diving bell on that little boat? That makes about as much sense as the pressurized tent that they pulled out in Moontrap.
The basic ideas of Waterworld (dry land drowned due to climate change, mutant guy with gills, search for last bits of dry land, bad guys in old ships) are all fine, but were ruined by lazy writing and a stupid urge to do 'cool stuff' that didn't make any freaking sense.
It didn't help that they spent absurd amounts of money on stuff that would have been better done by building a big pond somewhere.