I was in Maui in 2004 and we really liked the snorkleing at Black Rock. Black Rock sits right on Ka'annapali beach behind the Sheraton. We saw small sharks, turtles, eels and the usual naso, yellow and achilles tangs. The best part is you just walk from the beach into the water - and there you are. For a day trip go to Molokini, which is a sunken crater a mile or two off the shore. The clarity is incredible. There are numerous boats that will take you out there and most will also make a second stop somewhere else. Make sure you do your snorkeling im the morning as the winds pick up around lunchtime and decrease the visibility. We also tried ulu'a and napilli, but didn't like them as much.