The last time I was in Aruba, I was having dinner at the Flying Fishbone and was approached by a waiter asking me if we were interested in a snorkeling tour. We ended up meeting the 2 sweetest couples that basically took private groups out on their 4x4s. They had 2 cars and they basically told me that the most people they take out is 6 at once. When we went there was only one other couple so it was literally one guide per person.
We went to some of the less frequented spots, where the big tour groups didn't go. They served food and drinks along the way and it was one of the most memorable experiences ever. I think the men were Dutch, and one of the women was Venezuelan. I think the other woman was dutch. I think they were called "sky and dive" or "fly and dive". Does anyone know the group I am speaking of? I am going back to Aruba in 2 weeks and would very much like to contact them and go snorkeling with them again, if they still run the tours.
I'm sure I missed some, and others can chime in with their personal experiences with each company. Some may be more "commercial" (i.e. cattle-call tours) than you're looking for, so bear that in mind.
Another option, if you have your own gear or are willing to rent, is to simply shore dive. Some of the best spots are those not visited by the tour companies. If this is an option for you take a look at Shore Diving Aruba.
Regardless of what you choose, you'll have a great time!
I always like Jethro from the Octopuss, his boat is by Pelican Pier/Holiday Inn. He is very nice and owns his own boat. He does not overcrowd, and keeps things relaxing. Of cource he provides rum punch.