Here's more about it from KABC:
Reportedly, the explosives are not dangerous enough to blow up yachts or homes.
"They are commercial-grade blasting caps used to cause explosions like for mines and things like that," Jim Amormino of the Orange County Sheriff's Department told reporters. "The situation is very serious. They are concussion-type explosive devices, as opposed to fragmentation type.
"This was our concern, (that) they were attached to a bigger explosive device. There's probably six to eight or more out there.
"They are very unstable in salt water. They do pose a threat to the divers, any swimmers, any boaters in the area," Amormino said.
"We want to know how they got here, how many of them are out there,"
It just amazes me to hear stuff like this. If you were sailing a little 19' sloop right over six blasting caps in 10-12' feet of water, and they all went off simultaneously, odds are you wouldn't even know it. Sheesh. It would be unpleasant to be in the water swimming or diving near them if they popped, but only because of the way noise propagates in water.