Hi, i'm L. & i'm married w/ 2 girls. Emily is 6 1/2, & Sophie will be 3 next month. Your not alone. I have yet to figure out why Sophie does this. Never has she sat in the tub! Never. Even in the kiddie pools, won't do it. She too will yell & scream thru her bath routine. It's not to hot/cold. It doesnt matter what toys she has. Sometimes i can get her to sit, but only for maybe a full minute or 2. I've tryed the shower & that was worse. That was major meltdown. The 1st summer she was able to go in the pool she did the same thing, stayed on the outside edge & just splashed. I remember before she was walking i put her in it and she screamed & cried horribly. Even in my neighbors inflatable pool, it's like she's afraid of being submerged.
I havent been too worried about it. I try to work w/ her. Obviously it's a fear we can't force w/o to much trauma. So i just let her do whatever shes comfortable with. Chances are he may continue to be this way, unless he grows out of it. If not just go with the flow. :D Good Luck