Sharks use the effect water has on light to their advantage. In low light conditions, when the sun has recently risen or will soon set, light doesn't reach very far into the water. The light that does go deep is very distorted, which helps a shark to hide in otherwise clear water.
Sharks' backs have the perfect color for blending in during these conditions. From above, a Great White Shark's back is almost impossible to see in the right light. Marine Biologists have concluded that sharks hunt much more during the first two hours of sunlight, which helps to prove that they do this intentionally!