I don't recall seeing armed guards in Jamaica but there are definitely guards at the entrance to the Falmouth port who check that you are a cruiser before they let you in. Even at the resort we stayed at near Negril had gigantic tall doors into the parking lot with a bar on the inside and you had to request entrance but no one was armed. In Costa Maya there were armed guards - with hands kept on automatic rifles - that checked the buses going in and out of the port area including running a mirror under the bus carriage. That was in 2005 I think so I've no idea what it's like now.