The late-summer day was strange, and so was the water--thick and soupy, unruffled, dark as slate, yet with clear visibility a good 10 to 12 feet beneath the surface. It was also thoroughly alive. I cleaved through school after school of tiny, darting fish and through tens of thousands of golf-ball-sized jellyfish. Farther down, at closely spaced intervals, I caught the unmistakable silverish flash of something big and quick, swimming with authority, on a mission. I settled into a steady rhythm, mesmerized by it all.