You can't really say you have a private beach until you've installed a SM 2000 Underwater Surveillance System by Kongsberg to keep out the riffraff.
The system is designed to protect commercial piers, government and military vessels, cruise ships, terminals, and other high-value assets, but it'll work just as well for your hideaway surf break. You know it's good if the oil sheiks buy it. Kongsberg installed an integrated system at a "High-Value Seaside resort" in the United Arab Emirates; the exact location is classified.
The U.S. Coast Guard just picked up $2 million of Kongsberg gear to enhance its Integrated Anti-swimmer Systems (IAS) program at the nation's ports. The purchase follows the initial IAS contract worth $3 million.
Using software and sonar the system can detect and differentiate between "malicious swimmers and divers" and other targets, such as marine life and debris, at up to 1000 meters, according to the British Columbia-based company. A processor "captures a wide acoustic swath" to positively identify and localize the threat, then notifies security (PDF).
You'll be relieved to know that the Coast Guard and the EPA have concluded that the system will not "adversely affect threatened or endangered species or critical habitat." Whether a diver could do enough damage to justify the multimillion-dollar investment is open to debate.
Someone poaching in your favorite abalone patch? A frogman can be warned that he is in a restricted area and should surface immediately by "underwater loudhailer." If that doesn't work, deploy the "nonlethal interdiction acoustic impulse," an underwater shockwave emitter--which, despite its name, can be set on stun or kill.