The Taraporewala Fish Aquarium is located at Marine Drive. It has a fairly good number of fish and other marine life. It is popular among children. I remember it used to be on our list of destinations along with the Gateway of India, Juhu Beach and other places for the summer vacations when we were in school.
A proposal to develop this aquarium into an oceanarium (like Underwater World at Sentosa in Singapore), where people can take a walk in a tunnel and watch marine life, is under consideration.
Note: The aquarium is open on all days except Mondays.