Taken from "Answers.com"

The Enlisted Surface Warfare Specialist Badge (also known as the "ESWS pin") is authorized for wear by any enlisted member of the United States Navy who is permanently stationed aboard a navy afloat command and completes the Enlisted Surface Warfare qualification program and personal qualification standards (PQS). For those who possess "underway rates", that is to say enlisted ratings which may only be assigned onboard underway vessels, the Enlisted Surface Warfare Specialist Badge must be obtained after three years as a Second Class Petty Officer. Those failing to qualify are unable to re-enlist in their current rate. On each assignment to a new sea-going command, if Surface Warfare is the sailor's primary community, the sailor must re-qualify ESWS, though requalification standards are somewhat accelerated, and are often basically refreshers. Sailors for whom ESWS is their secondary community (mostly those in the Air Warfare community) are not required to requalify.

An enlisted person who has qualified for his or her ESWS Pin places the designator (SW) after his or her rate and rating; for example, Boatswain's Mate Second Class Jones, having qualified for his ESWS Pin, is identified as BM2(SW) Jones.

For those enlisted personnel who are subsequently commissioned as officers, and receive a Surface Warfare Officer Badge, the Enlisted Surface Warfare Specialist Badge is replaced since Navy regulations do not permit the wearing of both the Enlisted and Officer Surface Warfare Badge simultaneously. However, ESWS may still be worn if the officer does not earn a SWO badge, but rather a specialization in another community (ie, the officer becomes a pilot or Naval Flight Officer.

Unlike other warfare pins available to both enlisted and officers, the ESWS and SWO pins differ by more than just color (gold for officers and silver for enlisted is a common theme in U.S. Navy uniforms). The swords behind the hull on the SWO pin are sabers. The swords on the enlisted pin are cutlasses. This can clearly be seen in the curvature of the blades and the shape of the handguards. This derives from the saber being a symbol of naval officers and their authority, while cutlasses were traditionally the sidearm of the enlisted men.