I have a little 4 year old mutt. A few months ago I heard her barking in the bathroom. When I went to see what she was up to she was just standing in the bath. I said "Why are you in the bath?" and she jumped out and went and lay in her bed.
Since then she has done it 1-2 times a week. Doesn't always bark, sometimes I will just hear her jump in so I'll go and check on her and she'll just jump out and act like nothing has happened. Sometimes she will jump out before I even get to the bathroom and then she'll wait in the hallway and then run away as soon as she sees me.
The last couple of times (last Thursday, yesterday) she has jumped in the bath, I've gone to check, then she has run to one of her water bowls and started drinking. If I try to walk away before she has finished drinking she barks at me! I have to stand and watch her until she is finished and then she'll just go and sit on the sofa or in her bed.
She rings a bell if she needs to go outside so I don't think she's trying to tell me that. She also hates baths and getting washed so I don't think she's asking for a shampoo and style either.
If the bathroom door is closed she doesn't try to get in but DC is afraid of the bathroom door so it generally gets left open. There's nothing she can chew or hurt herself with, everything is stored out of reach, so I don't mind her standing in the bath if that's what she wants to do.