Looking at the Yankee Freedom website, it appears the ferry just brings people from Key West to Dry Tortugas. Once there, people are free to camp, see the fort, go to the beach, watch birds, or go in the water (to snorkel or whatever).
So the ferry is not a "snorkel tour" boat. It brings people to the island and the people are free to plan their day. The crew may have suggested where it was best to snorkel, which could have been "in the water, near the shore" if these two people were so "very inexperienced."
But suing the ferry for negligence just because they brought the victim to the island makes as much sense as suing the airline that flew them to Key West because the flight attendants failed to give the passengers a full briefing on safe snorkeling procedures.
Hopefully the case is tossed.