She is simply afraid. (two year olds do that sometimes) Ask her if you lower the water really low if she will sit or even stand in the water.
Start by lowering the level of the water to about 2 or one inches. Ask if that is okay with her. If is is too high, lower it.
Let her do that for a couple of weeks and if she sits rather that stands, then she is really relaxed and having fun, gradually raise the level of the water 1/2 inch. If she notices and freaks out, put it back.
The goal is too have her relaxed, not to take a bath in deep water. If 1 inch is all she can do, that is all she can do. Give her six months and it will be a different issue.