Submersibles, as these submarine-like craft are often called, are drug cartels' secret weapons, according to Rear Adm. Joseph Nimmich, an adviser for the Joint Interagency Task Force.
"They're very hard to find. You don't get any radar reflection off the type of hull that's here," he told ABC News. "It runs almost submersed; visually, it's hard to see."
At left, a submersible is caught cruising the waters. (ABC News)