Just be aware that the boats from the state park go to the nearby heavily snorkeled reefs, which are (not surprisingly) in less than stellar shape. Each boat (they usually run 2 at once) carries 50+ snorkelers, so there are a ton of people in the water.
Other private charters in the area take fewer people, and go to (IMO) better spots. My wife and I like to go out on the Morning Star. They do a 4 hr sail/snorkel trip with a maximum of 6 people, and stay far away from the big boats. It's $65 per person, compared to ~$30 from the state park. But, sometimes you get lucky - the last time we went out, we were the only ones on the boat.
If you've never been on a real reef before, you should go, just for the experience. You would need a car, there's no other way in/out of the keys.