This natural phenomena is caused by phytoplankton (a type of microalgae that float at the surface of the ocean water). They emit a bright blue light after they become agitated by wave movements or nearby swimming fish.
Phytoplankton have channels to allow protons (positively charged subatomic particles) to pass through their bodies. So, the nearby movements in the water cause protons to pass through their bodies, creating electrical pulses, which trigger chemical reactions. These reactions, in turn, activate a protein called luciferase, which creates the blue light.
Bioluminescent phytoplankton are almost exclusively found in salt water. This is believed to be the case because one of the chemicals needed for the reaction isn't found in freshwater.
If this sounds to you like a sight that you don't want to miss, here are some places where you can experience this magical phenomenon.