We booked this trip with Trilogy (http://www.sailtrilogy.com/?source=goto). I was very impressed with their professionalism blended with friendliness.
We met at the Ma'alaea Harbor at 6:15am. The catamaran was fit and clean. There were about 50 folks on the boat, but it wasn't crowded (I understand some other tours take up to 100+, makes for crowded diving, if nothing else). We started off with a brief safety speech from the captain, coffee, juice, fruit, and fresh cinnamon rolls. Then we were off!
We were exceedingly fortunate to have a rare calm morning out, and it stayed warm even though we wore little more than swimsuits and tanks tops at 7am.
Soon we arrived at the crater. The captain gave us a short history/nature lesson, and told us to be sure to not touch the delicate coral, not to go past the curved arm of the crater, as the Hawaiian name for that particular current translates to "Go To Tahiti"! Then he gave us new snorkelers the basics, and lowered the stairs. I was more than a little nervous, but very soon I didn't want to take my face out of the water! NOTE: Put water-proof sunblock on your back *and* on the back of your legs!
I can't remember the names of the fish we saw, but there were lots of black hand-sized fish with backwards-looking fins so close to the surface that once you knew what to look for you could see their little dorsal fins protruding fron the water like hilarious little sharks! I know I saw a boxfish, possibly a small octopus, screaming yellow flat fish (my own name for them), and many others. It was so fun to float above the clusters of fish just a few feet away in the crystal clear water, minding their little fish business, when a stronger wave would crash around us and we'd all get pushed around back and forth, eventually ending where we began. After some time (hours? minutes? It went fast) we hauled ourselves back on the catamaran and sailed over to McKennett's (sp?) Bay, home of the Green Sea Turtles. Another quick history and legal lesson (FYI, it's a $20K fine + possible prison time to touch the turtles) and we were back in the water.
The turtles were very friendly and accommodating that day. They were especially friendly towards my husband, who had to swim backwards repeatedly to avoid the forbidden turtle contact. One even dove immediately below him and stayed there for a bit.
Eventually the scent of BBQ-ed chicken lured us out of the water and back on the boat. A simple and very satisfying lunch ensued, and afterwards we enjoyed a leisurely sail back to the harbor.
If you can do only one thing in Maui, do this.