Dear L.,
Geez, I'm sorry everyone's giving you such a hard time about bathing every day! Clearly that was not the point of your question, nor the source of your problem! I only bathe our almost-3-year-old daughter every 2 or 3 days because she has very dry, sensitive skin, but I insist on washing her hair every time, and she has absolutely hated that part since about 18 months of age. (Screaming, crying, pushing, thrashing, etc!) She'll use a cup to pour water on her bath toys' heads and say "wash hair," but if I pick up a cup, she freaks out.
Nothing helped much until recently when she became old enough to understand the concept of future rewards. I'm not generally big on "bribing" kids, but in this case, it seems worthwhile. One night I explained to her that we were going to wash her hair, as usual, because that's part of taking a bath, but that if she was nice about it, she could get a treat after bath was over. (A Skittle or Vitamin C gummy or similar...) However, if she was not nice about it, and screamed and cried and made it hard to wash and rinse her hair, I would still wash it, but she wouldn't get a treat. She agreed that she wanted a treat, and was surprisingly agreeable throughout the process. (She still hated it, but the thrashing was milder, she didn't scream, and she let me do it!) It's worn off a little since the first few times, but in general, if she starts to get awful, I remind her that she has to be nice if she wants her treat, and she settles down. (I also still try to get the whole hair-washing part over as fast as I possibly can.) We make a big deal about the treat, and she wants to show Daddy her treat, and I tell her I'm so proud of her and she's a good helper.
I'm sure at some point the novelty will wear off, at which time I think I'll try that "bath hat" idea. (Nice one!) You could probably do a whole star chart thing if your son was into that kind of thing -- whether it's treats or stars, I think encouraging cooperation is a useful skill. Oh, and swimming has had no effect on the whole thing. She loves the pool and the ocean, but she's still scared of water in her face, and she still despises hair washing.
Best of luck to you! I just try to remember it won't last forever... but right now it's SO frustrating! (And if her skin could handle baths more often, I would definitely give them to her -- I don't know about everyone else's kids, but Katie gets very grimy very fast!)
- L.