Question

how was water filled in great bath in comparison with the present day swimming pool

Answer 1

it fill with naturally water coming from the ground

Answer 2

Wells is the right answer.

In the Indus Valley Civilization, the Great Bath of Mohenjo-Daro is of great importance. Water in this tank was filled by the people from wells. On special religious functions, people took bath in the Great bath. Two broad staircases reach down into the tank from the south and north and small openings at the ends of the stairs are believed to have continued wood boards. People maintained cleanliness and soon after using Great bath the water was drained out.  

In the Modern day swimming pools, water is filled with the help of a garden hose. It is a flexible pipe to carry water. It is used by placing the end of the hose on the bottom of the receptacle (the inward end of the pool) and by turning the water on.