We went to Maui several years ago. We went out on a whale watching/snorkeling sailboat from just outside of Lahaina. Left at 7am and got back at 2 or so. There were about 20 people on it, max. They have bigger boats with up to 100 I think, but we wanted to have less people.
Anyway, we went out and they gave us breakfast then we looked at whales. Has to be the right time of year for that. After that we went to 2 or 3 places to snorkel. One was with giant turtles. The others were places 15 or 20 feet deep to look at fish and coral. At one of them they threw some food in the water and there was a huge school of Niger triggers that we got in and swam with. On the way back we got lunch. It was by far my favorite thing we did. So cool!