Dr. Greene's Answer
As kids’ imaginations blossom, so do their fears. Often the easiest way past them is with little steps. With bathtub fears, often the best first step is for you to get in the bathtub alone and let her lean in and play with toys without getting all the way in.
Another thing you can do is get in the tub with her and play with no water from the faucet–perhaps just a bucket of water handed to you. Then gradually have a very thin layer of water on the bottom and increase water levels over a period of a few weeks. Or, let her be dressed in a swimsuit or other clothes and gradually decrease that. The key is to find the border that is comfortable for her and then not ask her to make a leap, but just the tiniest step forward. When that is comfortable, she can take a tiny step forward again.