Though it's been many years (my babies are in college) this is precisely what happened with my daughter. She loved bath time and then, all of a sudden, she was terrified and I couldn't link it to anything at all. We backed right off because she was so clearly afraid. I still ran her a bath, asked her if she wanted in, suggested calmly that it would be fun but then said, "well, let's just get washed off then" and gave her a sponge bath. That was fine with her but by day 3 I was a internally thinking that she was starting to really need a bath...
Right then, Mr. Rogers (RIP) came on with an episode where he calmly sat on the lid of the toilet, splashed his hand in the bath water and sang" No, you'll never go down, you'll never go down, no, you'll never go down the drain." Abby was completely transfixed by this segment so when it was rebroadcast that afternoon I videotaped it (right. years ago.).
So, laugh if you will (I did and I'm a child and family therapist, not of Freudian ilk) but that was the issue apparently. All I know is that she watched it about 100 times over the next day or two, I said "that's right, you're too big to go down the drain" and then she matter-of-factly started taking baths again.
I have no idea where you would find that old episode or whether that has anything to do with what's going on, but we've often laughed about our debt to Mr. Rogers. You may need to just find a way to do a demo and then delay draining the tub after bath time for a while.
BTW- my daughter is now a master diver majoring in marine science. :-)