I remember reading about this some time ago. Taken from http://en.wikipedia.org/wiki/Milky_seas :
Milky seas is a condition on the open ocean where large areas of seawater (up to 6,000 square miles) are filled with bioluminescent bacteria, causing the ocean to uniformly glow an eerie blue at night...
... conclusions were that the effect was caused by the bacteria Vibrio harveyi.
http://en.wikipedia.org/wiki/Milky_seas :
Milky seas is a condition on the open ocean where large areas of seawater (up to 6,000 square miles) are filled with bioluminescent bacteria, causing the ocean to uniformly glow an eerie blue at night...
... conclusions were that the effect was caused by the bacteria Vibrio harveyi.