To the best of my knowledge, there is not anyone that does that. There are not any operators except the resorts.
LCBR rents small catamarans for use in the lagoon, but there is not much snorkeling there. I have not seen any other catamarans on the island.
There is what I would call a patio boat near Southern Cross Club. If you could rent that I think that is about as close to it you can get.
The underwater topography of the island does not really lend itself to what you want. Going east from LCBR you would have to travel to near the eastern end to snorkel and going west, you would have to travel to just around the west end. All of this of course assumes a fairly calm day.
LCBR also rents a small fishing boat with Captain.
It is just not simple answer in my mind.
Edit: Actually, it is a liitle closer going to the west through the cut to get to a snorkeling site. You might also consider going to Point of Sand. It has a great beach, very few people and the snorkeling is outstanding over a large area. Take a lunch too. No catamaran as there are two cuts you would have to traverse, in addtion to the distance.