Freshwater Underwater Clicking Noise. Plankton??

[SmithDiving]

I have never noticed this in inland freshwater lakes, but almost always hear it in the ocean.  I've heard it in Micronesea and the caribbean.  I have aways heard these sounds and assumed it was just the environmental sounds that we don't normally hear (as we hear so much better underwater).  I would guess that we are hearing a portion of the marine life sounds along with any  man-made sounds like boat motors and underwater cables (electrical/phone etc.).  I'm sure that when you hear the clicking/static-like sounds underwater, you are just hearing a portion of what all the critters hear all the time.