Heading to the Keys for our first trip and staying "central" in Marathon. We are a family of four and BIG snorkelers, having been to Cozumel, Belize, Grand Cayman, etc. All of these places offer "walk out" snorkeling that you can find yourself either via a beach bar or public beach access. Are there any places in the Keys that offer locations where we can park and snorkel or do we need to take a charter? Thanks.