Giving a cat a bath once in a while is both necessary and dangerous. If you don't mind the bloodshed, it's also hilarious. If you've never had the fortune of cleaning a cat, think of it as washing a bar of soap that's covered in cactus needles. And razor blades.
About 70% of the Earth's surface is covered in water. Humans are 65% water. Cats are 65% water. They drink it. They throw their toys on it. They'll sit out in the rain. But if you put them in a tub with two inches of water in it, oh man, they will hurt you. And should you survive the aforementioned ordeal, they will spend days plotting their revenge. Or until their fur dries and you give them some tasty noms.
To be fair, if I looked like this when I got out of the shower, I'd probably be out for blood too.
My theory is that cats have a terrible phobia of large bodies of water. Understandably so, because you can drown in water. Planet Felinus (or whatever you want to call it) is only made up of about 25-40% water, making it significantly easier for cats to avoid anything water related besides little puddles and rain. This enables the phobia, because it didn't give the Original Cats a chance to get over the fear. If many, many generations of cats never see oceans or ponds or bath tubs, that fear will become ingrained into the general personality of cats.
Like all unique creatures, different cats handle this phobia in different ways. Some take the cautious, curious road, such as watching the water faucet for hours on end or staring at you while you take a bath in utter fascination. Others act on their phobia more aggressively, yowling angrily at the sky to make it stop raining or trying to "kill" the bowls of water they're supposed to drink from. No, seriously. We have a two pound rock sitting in the cats' drinking bowl because one of the cats kept tipping the water over and spilling a liter of it on the floor. The laminate wood floor. With plenty of grooves for the water to slip under. Oh, whatever. He sure showed that bowl of water who was boss. Again and again and again.