One night I picked them up and they glowed at the tip of my index finger, bright enough to actually see the minute detail of my finger.
These are called bioluminescent plankton. They glow because of the luciferins in their body. The light is produced by a series of oxidation reactions set off by a catalyst called luciferase. The interesting part is, they glow when they are agitated, or when they sense perturbation in their surrounding. For example when a squid swims it induces current which agitates the plankton, and thus they glow. That's how predators and prey detect each other's presence in the ocean at night.
|Dinoflagellates, bioluminescent phytoplankton. Image: ux.brookdalecc.edu|
That wouldn't work against the Blue Whale though.
So if you happen to go diving or snorkeling at night, turn off your light and wave your hands through the water. And watch the most primitive form of lighting.