The Bathroom is one of the rooms in Croft Manor. When you begin exploring the Training Level, you have to turn around to the left to find the bathroom behind you. The bathroom is adjacent to Lara's Bedroom (TR3) and is right next to the Closet.
The bathtub is filled with water but not enough to swim in it. Lara can wade through it and in case she got to close to the fire in the fireplace, she can jump in to extinguish the fire, although the might not make it in time as she loses Energy quickly. When looking out the window you can see the Assault Course.