I was just in Maui earlier this July and I agree with what the others above have stated.
I can also confirm that the "Fishbowl" and the "Aquarium" have been closed to the public due to excessive damage to the reef from tourists. The state has now directed all snorkeling and diving in south Maui to a new area aptly named "The Dumps" which is just north of the Aquarium and Fishbowl with much easier access. I only snorkeled there once but I wish I had gone more times as there were different species of fish and very tall coral mounds present that I didn't see anywhere else around Maui. There was also a large pod of dolphins spotted at the dumps daily.
Molokini is definatley worth the money to go check out. Underwater visibility is often greater than 100 feet and it was the only place in Maui that I saw Achilles tangs and blue throat triggers. Probably the healthiest reef there as well but not nearly as much structure as the Dumps or Honolua Bay. If you hit up a "Tickets on the Rock" booth in any one of the towns in Maui (there is at least a half dozen in Lahaina alone) you can get a better price on a trip out to Molokini. Don't believe the signs out front though as the only way you can go to Molokini for the $18 advertised price is to sit through some sort of painful time share.
I also snorkeled Honolua bay every day except for one while it was raining. Definately worth checking out, especially if you are staying in the Honokawai area. Don't bother going here if it rained alot the previous day as a creek takes all the runoff out into the bay leaving a 12-18" thick layer of silty and cold fresh water on the surface of the bay reducing visibility to near zero unless you are diving.
Parking at black rock is terrible as it resides infront of the Sheraton Hotel. I tried to go there on several occasions and was unable to find a parking spot in the free area (only 6 stalls) or in the mini mall parking lot. Go very early if you plan to snorkel this area.
Here's a link to some pics I took while I was there:
http://www.canreef.com/vbulletin/showthread.php?t=76942
Enjoy your trip!