Spotted this article by Barry Fox on NewScientist.Com:
Spy-diver killer

If you are scuba diving, be sure not to swim anywhere near any ship or
installation that has been protected by the Raytheon Corporation's new
"swimmer denial" system. Otherwise you will very quickly feel extremely
sick and probably drown.

Raytheon's underwater sensors detect any unwelcome presence and
trigger an underwater sound system that emits extremely powerful pulses
of low frequency audio. The pulse rate and audio frequency are chosen
to make human organs resonate like organ pipes, causing swimmers to
vomit into their masks or suffer internal ruptures.

The idea of blasting very powerful sound at underwater targets is not
new. It can even be used to detonate incoming torpedoes. But it can
also cause havoc with marine life.

Raytheon's new system is "greener" because the main sound projector, in
the middle of the secure zone, emits sounds with power and frequency
that are relatively safe. A dozen or so secondary projectors in a ring
round the zone also emit safe pulses. But in the region near each
secondary projector the main and secondaries combine to produce a sound
which is decidedly dangerous.

Would-be spies or terrorists cannot get through the ring but there is
no widespread danger to fish, dolphins or whales.