"...But perhaps the ultimate security measure a ship owner can adopt is the use of armed security personnel, either provided by their government as Vessel Protection Details (VPDs) or through employing Privately Contracted Armed Security Personnel (PCASP), where Flag state rules allow. The latter are often made up of former members of various armed forces, who embark on merchant ships and guard them during transits through high risk waters. To date, not a single ship with armed security personnel aboard has been successfully hijacked. These teams have served as a game-changer in the effort to combat piracy.
For our part, the U.S. government has mandated that U.S.-flagged merchant vessels transiting the high risk area conduct a risk assessment with specific consideration given to supplementing onboard security with armed personnel.
When PCASP emerged on the scene a few years back, there were reservations. Many feared that armed security personnel would escalate the level of violence during pirate encounters, further endangering mariners. The opposite appears to have happened. From the evidence that we have seen, in most engagements, the attack ends as soon as pirates realize an armed security team is on board. Pirates often break off their boarding attempt and turn their skiffs around to wait for another less protected ship. These teams therefore have served as an effective deterrent.
However, PCASP teams come in varying sizes and, to be frank, in varying degrees of quality. Their emergence as a security option has brought with it complications. Varying national legal regimes complicate the movement of these teams and their weapons from ship-to-ship or ship-to-shore. Some flag states do not have clear legal guidelines for addressing armed security personnel and are struggling to formulate positions vis-à-vis armed security personnel at sea.
Untangling legal and policy issues related to armed security will take time. But the U.S. government is hoping to make progress. Last month, the U.S. Department of State hosted a working level meeting of policy specialists from 23 nations and international organizations. The intent of the meeting was to give participants an opportunity to share information about their national or organizational law and policy on PCASP, thereby allowing all involved to gain a more complete picture of the overlaps and gaps in legal regimes and policies from country to country. This is a crucial step in figuring out a way forward that addresses the thorniest differences.
As a legal matter, authority over the use of privately contracted armed-security personnel beyond territorial sea limits (12 nautical miles from land) falls to the flag State. Once a vessel with armed personnel embarked enters territorial seas it may carry such personnel provided it is engaged in innocent passage or transit passage. If a vessel with an armed team embarked intends to enter a port, the port State may exercise authority for regulating the personnel or their arms."
http://m.state.gov/md199929.htm