Welcome to the Board, Steve. Snorkeling in San Carlos is hit or miss. You can actually get pretty decent snorkeling out at San Pedro Island, but it's a 2 hour boat ride out there. San Carlos doesn't have much as far as shore diving/snorkeling. The local shops down there do sell snorkeling spots on their boats. Believe it or not, San Carlos is actually more of an Arizona Scuba area than a Mexico area. The majority of divers in San Carlos drive down from somewhere in Arizona, so this would be better suited in that forum.
Steve, you can also take a look at the threads at the bottom of this page and click on them.