Unlike many other islands, you'll find most group tours are focused around trips to a single destination, that being Sting Ray City. There are some snorkeling specific tours to other locations, but they are not full day tours. Private charters are expensive, but they sound like more of what you're looking for.
For a snorkel only tour (not including Sting Ray City), Ocean Frontiers offers a great snorkeling experience from their East End Dive facility. I've been twice and never had over 10 other snorkelers, but its only a few hours.
There are plenty of private charters to choice from. Some to look at are Mainstays Sailing and Aquarius tours, but be prepared for potential sticker shock if its just the four of you. There are power boat tours available as well, that may be a little less costly. Just be sure to confirm whoever you are looking at is a member of the Cayman Islands Tourism Association (CITA)
You should take a tour to SRC, and I'd highly recommend you make the effort to take an early morning/dawn tour to enjoy a much more intimate experience than a mid-day or afternoon tour. I've done it with White Sands Water Sports and can highly recommend.