There are two types of boats/launches : The Luxury Type and Economy Type. The former cost Rs150 while the later cost Rs120 for return ticket. If you have bought the Luxury ticket, you can return by either services based on the your choice. They collect Rs10 inside the boat to sit on the upper deck. It’s worth this ‘premium’ as you can get a good panoramic view, the crowd and noise is relatively less.
The ferry timings to Elephanta : The ferry travel time is about an hour. The first boat leaves from the Gateway of India for Elephanta at 9 am. The last one for elephanta leaves at 2pm. And typically there is one leaving for Elephanta every 30 minutes, between 9 am and 2 pm. Also note, Elephanta Caves are closed on Mondays. So is the ferry service to Elephanta.
Similarly, the ferry schedules from Elephanta are like this: the first return boat leaves Elephanta by 12 noon. The last one at 5.30 pm. And you'll find a return boat every 30 minutes in between.
If you are looking for a fastest whirlwind trip to Elephanta catch the first boat to Elephanta at 9am, you'll reach the island by 10 am, another 30 minutes to reach the caves (10.30am). About an hour or so later you can return to catch the earliest return boat. Weather you make a quick return or not, take the first boat, you'll enjoy the ambient at the caves, sans the thick crowd that keep arriving by the subsequent boats!
Another advantage of going early, you can leisurely explore the other attractions in Elephanta like the Cannon Hill and those minor caves.
Or if you are late for the day, catch the last boat by 2pm, arrive at Elephanta by 3pm, another 30 min trek to the caves site. You'll have another 1 hour or so to spend at the caves. Decent at least by 5pm so that you'll not miss the last boat back to Mumbai!
BTW , the boat ride per se is not spectacular , expect that you'll get a fine view of the Mumbai Harbor and the Gateway of India with a the Taj Mahal Hotel adjacent to it. You may also pass a few massive tanker ships moored and the Butcher's Island with a whitewashed fort on it. See also How to Reach Elephanta.