Water moccasins (aka Cottonmouth) are carnivorous and will eat whatever they can catch. They are a venomous water snake found in the southern United States. According to an Internet search, their food sources include small mammals (rodents), reptiles (other snakes, lizards, baby alligators, turtles), frogs, fish and birds. They can be very aggressive if they feel threatened (personal experience) and people have died as a result of their bite.
According to Live Science, water moccasins eat other snakes (including smaller water moccasins), turtles, baby alligators, birds, amphibians, reptiles (lizards), small mammals, and fish.
Insects are not on their diet and they do not eat them.