We visited Aruba for a week a few years ago and did quite a bit of snorkeling there. The good snorkeling spots in Aruba are right off the beaches, so unless you want to do a tour for the convenience of it, you could just as easily take a taxi and save a lot of money. We would be snorkeling and a boat would pull up right there, and 30 people would get out and snorkel for 45 min and then have to leave. (One tour operator yelled at my mom to "come get back on the boat!"
Anyway, north of Orangestad and the hotels and just before the California lighthouse is a good snorkeling area, and probably where a tour would take you. We snorkeled there several times.
Another spot is at the south tip of the island, at Baby Beach. It is a beautiful beach, and the snorkeling is okay, but if you go out past a rocky barrier into the rougher water, there is some amazing snorkeling. It was a little intense though, and probably not everyone would enjoy it in the rougher water. But we saw some amazing fish/coral.
The only other good spot I know of is the Natural Pool, which is very neat, and you'd probably have to take a tour there (we rented a jeep....it is pretty off-road to get to). The snorkeling is not as good as the other spots IMO, but you have the experience of the pool with the waves crashing over the top. We took some beautiful pictures.