"The Great Bath, which I have reserved to the last, was part of what appears to have been a vast hydropathic [water therapeutic] establishment and the most imposing of all the remains unearthed at Mohenjo-daro. Its plan is simple: in the centre, an open quadrangle with verandahs on its four sides, and at the back of three of the verandahs various galleries and rooms; on the south, a long allery with a small chamber in each corner; on the east, a single range of small chambers, including one with a well (no 16); on the north, a group of several halls and fair-sized rooms. In the midst of the open quadrangle is a large swimming-bath, some 39 feet long by 23 feet broad and sunk about 8 feet below the paving of the court, with a flight of steps at either end, and at the foot of each a low platform for the convenience of bathers, who might otherwise have found the water too deep. The bath was filled from th well in Chamber 16 and possibly from other wells besides, and waste water was carried off through a covered drain near the S.W. corner, the corbelled roof of which is some 6 ft. 6 in. in height( Pl. XXIV, a, b and c)." (John Marshall, Mohenjo-daro, Volume I, p. 24).
What do you think the Great Bath was used for? Iravatham Mahadevan has an unconventional idea in The Indus Fish Swam in the Great Bath.