I suppose a water "carrier" could have a sieve to filter away unwanted particles, or could add chlorine to kill bacteria, etc. But it certainly couldn't do anything equivalent to keeping or selling logs...
Well, they could drugtest your wastewater [0] and measure your water consumption with one of these "smart" meters at small intervals. It's not at the scale of data that an ISP could sift through, but with a little imagination, water can be a source of metadata...
