It was a traveler’s worst nightmare: on May 31, a half-dozen
hooded pirates waving grenades and firing assault rifles into the air
attacked the ferry making its evening trip to Playa del Carmen from
Cozumel. According to police, the privateers were targeting a money
shipment from Cozumel’s businesses that was en route to mainland
banks.
The incident did get passengers’ attention. Vacationer Tracy Miller, a 33-year-old Los Angeles postal
  clerk, reported hearing gunshots before the boat stopped and said “we didn’t know what the gunshots
  were; we didn’t know if they were shooting other passengers.” The ferry’s engineer was badly beaten, and
  two security guards who were transporting the cash were thrown overboard before the robbers sped off in
  a speedboat. One guard drowned, but his companion was rescued the following morning. Local reports
  said that the surviving guard, who, unlike his deceased partner, was given a life jacket before being thrown
  overboard, has been detained by police. While the police have detained suspects in the robbery, at last
  report they had not recovered the missing funds.  
The episode appears to be an isolated incident, but shows once again that when you travel, you should
  be prepared for anything.