DD did a similar thing, and is now still scared of the bath if near the plug.

I put DS's bath seat over the plug and she is fine, but if she can see the plug when she is in the bath, there are tears, trying to get out and no reasoning.

She went through a phase also in the shower where we couldn't put her on the floor, had to hold her the whole time, so I am just assuming that she was scared of getting sucked down with the water.

Prob not much help, but it does seem to be as passing phase that most kids go through.