April 29, 2012:
South Korea is buying an unspecified number of Cerberus Diver Detection Systems to protect its major harbors from attack by terrorists or North Korean commandos. The Cerberus Diver Detection System is a (large) refrigerator sized device that is lowered to the ocean floor in the middle of the area you want to guard. Cerberus uses sonar to detect anything large enough to be a threat, up to 1,000 meters (or more) out. Cerberus works well in shallow water and can be tweaked by the operator, once emplaced, to be even more accurate.
North Korean commandos are known to have scuba gear and are trained for underwater attacks like this. South Korea is not specifying how many Cerberus systems it is buying or where exactly it is placing them. That kind of information would enable the North Koreans to plan attacks on port areas where Cerberus wasn't.
When hostile divers are detected there are also underwater weapons available. An American firm developed a sonar type device that can disable divers by using sound waves that are tuned to cause severe gastric distress in humans. Makes you heave into your scuba mask. This makes further underwater operations difficult, if not impossible. However, this system has not yet been made available for purchase. There may not be a large enough market to justify it. For all the talk about scuba equipped terrorists, there have been very few cases of this sort of thing actually happening. Most have been off Israel, where the Israelis have been able to deal with the problem without using an acoustic swimmer denial system.