Organizing sailors for ground combat is an ancient tradition. In World War I, there were entire divisions of sailors in the trenches on the Western Front, and in World War II, some navies still took a fraction of ship crews, armed them, and put them ashore under the supervision of marines, to fight on land. This happened as recently as the Vietnam war, where sailors served in the Brown Water Navy in the Mekong Delta, and often found themselves fighting on dry land. And then there are the Navy SEALs, a small, but potent force of sailors who are fighting mostly ashore against terrorists all around the world.
The U.S. Army is supplying most of the combat troops for the war on terror, and is stretched pretty thin as a result. The air force and navy have been helping out. In a rare move, the navy is supplying 600 military police to replace army MPs guarding captured terrorists at Guantanamo, in Cuba. The navy doesnt actually have MPs, but they do have thousands of sailors trained as "master-at-arms". These sailors operate somewhat like MPs, taking care of shipboard security at sea, and in port. Two years ago, the navy decided to more than triple its master-at-arms force to over 10,000 sailors trained for this duty. This was done mainly to improve security for ships in port, especially foreign ports. Thus the navy was able to assemble 600 of these naval security troops for guard duty at Guantanamo. The sailors went through additional training so they could handle the hard core terrorists who comprise a large fraction of the prisoners held in Cuba. The army MPs can then be sent to places like Iraq or Afghanistan, where the terrorists are armed, and even more dangerous. Army MPs are trained to deal with that, as well as guarding prisoners.