I agree, most of the movies he stars in suck, but its mostly cause of a lousy script or a hopeless idea of a movie.
The reason Waterworld tanked was mostly because of a trend that was starting at the time, real bad reviews for no reason whatsoever, and at that time most of the ppl that went to the movies relied on reviews. Now a days ppl depend more on word of mouth, than actual reviews. But at the time Waterworld came out the reviews simply killed the movie. Now the movie itself was really good, specially on the Special edition DVD, where the deleted scenes reveal some plotholes, the biggest one being the actual location of Dry-land.