L.,
my son did this too... after about 3 weeks of sponge baths and washing his hair in the sink while he laid on the counter, i knew we had to do something.
I absolutely can respect and understand that they have fears, and sometimes the fears they have aren't rational (like fear of going down the drain like the water - this is very real to them... but we know it's not possible).
I have to admit, i pushed my son.....slowly, but i still pushed him. he too seemed genuinely frightened... he would actually tremble and try to climb me to get away from the water... he wrapped his legs tightly around me and squeezed me so hard with his hands that he left bruises on my upper arms.
We started out with him sitting on the edge of the tub, and i was holding him. we would splash his feet - he cried... i kept doing it, telling him it's ok... mommy won't let you be hurt (they understand far more than they can verbalize).
Then once he was ok with that, it took about 3 days for him to actually enjoy his feet being splashed. we moved on to splashing other parts. Eventually i placed him in his bath seat, even though he was fighting me a little... i kept reassuring him that he was ok, and mommy wouldn't let him get hurt. We didn't focus too much on actually getting 'clean', we played... i put lots of baby bath in the running water so there was a ton of bubbles... gave him some cups to play with... we worked it up to the point where we were able to dump water on him up to his shoulders... he was constantly reassured, and i honestly dont believe he's suffered any emotional damage from working past his fears. Now he hears the word bath and he goes running upstairs and wants to get in the tub.
we did discover though that his fear developed while I was in the hospital having #4. I was there for 3 days recovering from a c-section and during that time my husband gave him a bath. Now by no means do i think he did anything 'wrong', but he isn't as seasoned as I am, and does things differently than I... so maybe my son just simply didn't like the experience...