Here is a set with Tania Aresti in the Lake District. I had rented an Airbnb house with this bathtub (one of the reasons why I rented it). After taking several outdoor shots with Tania, she was cold. So I suggested we make a shoot in a hot bathtub.
You would not imagine the amount of enthusiasm :D
The light is entirely from the window behind the bathtub.
The main things to look out for are to have excellent light on the face of the model and to look out for unsavory shadows.