We did a fantastic trip with Kubuku Dive Center.
Every day 100 boats leave the harbor of Banyumandi to go snorkeling / diving around Menjangan Island. Probably most dive centers organize similar trips and probably you will see the same coral and fishes (and maybe even an turtle). The reefs around Menjangan Island are fantastic, and these reefs are in my personal top 5 "best snorkeling places in the world".
But what I experienced at Kubuku is that Kubuku is very well organized and took care of all the guest during the day.
We had two boats, all snorkeling equipment fitted (and the had a lot of equipment), good transport from Pemuteran to the harbor, good lunch & enough water and even clean towels for everybody. We did two snorkeling tours, with a lunch on the beach and a relaxing swimming / sunbathing breach in the middle. In our boat there were 6 children from different ages and they could all do their own snorkeling tour, from 15 minutes to the full hour. If you wanted to leave the water, one of the instructors would arrange a pick-up by the boats.