When I was a child, my eldest brother (5 years my senior) almost always took a book with him when he went for a bath, especially in the winter months when it would be too cold in the unheated upstairs bedroom for reading. We were a big family, and I think he was just using the bathroom as somewhere he could read in peace and quiet. However, he couldn't stay too long, as the hot water tank depended upon the back boiler from the living room fire, and the bath water would cool down eventually.
Some time in the mid-1950's mam found the money to have an electric immersion heater installed. Worried about running up a huge bill for the electricity, she warned us all that we weren't to use it unnecessarily, or leave it switched on too long. Suddenly my brother's time spent in the bath tub started to become much, much longer. Indeed, once or twice I even spotted him taking two books in with him.
When the electricity bills suddenly started to get a good bit higher than expected, mam realised that he was simply leaving the immersion heater turned on whenever he went for a bath. And, if the bath water cooled down too much, he would simply pull the plug to let some of it drain away, and then top up with more hot water.
Mam finished up paying an electrician to move the switch for the immersion heater from inside the bathroom to the living room!