What's the smell that instantly takes you back to the ocean?

I was walking the harbor this morning before the fog burned off, and the low-tide smell hit me harder than any memory I've had in months. Not the beachy salt-air thing — the real one, the mud and diesel and whatever's rotting in the pilings. Curious what smells do that for other people. Not asking for your best beach vacation, just the one that actually sticks.