Depth sounder alarm in San Pedro Channel . . . ?

[Playa de las Almejas]

I've usually found the 'unexpected shoaling' phenonema to be echoes from passing over a school of baitfish, raft of submerged seaweed (yada).

A fishfinder will display the characteristic blob shape associated with bait balls and gives a bit more information than a simple digital depthmeter.

If you see something with 'hard edges'... heh, then you've got a problem.