When I was growing up, I always wanted a cat despite the fact that half of my family is allergic to them. I recently saw a video of a guy trying to bathe his extremely reluctant cat and it made me wonder if all cats really hate water as much as we think they do.
This article discussed why cats aren't big fans of water. A lot of people are under the preconceived notion that cats automatically hate water so they don't often expose them to it if they don't have to. A lot of times, a cat's first experience with water is a bath they are being forced into or getting caught in the rain so it's not surprising that they wouldn't like it. Scientists believe that a cat's dislike of water comes from their ancestors. The wild cats of Europe, Africa, China had limited experience with water and didn't have to adapt or evolve to deal with rain or water.
According to another article I found, a possible reason that cats don't like water is because their fur becomes waterlogged and weighs them down. It causes the cat a great level of uncomfort due to its coat being drenched. Cats can also be sensitive to odors so it's a possibility that the cat may not like the smell of the chemicals in tap water. Here's an actually highly amusing video that I watched of a guy trying to give his cat a bath: http://www.youtube.com/watch?v=eFk0kAwLiEk#t=114