Mark "Onmark Productions" Schumacher has published an insanely detailed introduction to Benzaiten Imagery in Japan.

Prior to the 12th century, Benzaiten's Hindu origins as a water goddess were largely ignored in Japan. But sometime during the 11th-12th centuries, the goddess was conflated with Ugajin (the snake-bodied, human-headed Japanese kami of water, agriculture, and good fortune). Once this occurred -- once Benzaiten was "reconnected" with water -- the level of her popularity changed from a trickle into a flood.

(iseewhatyoudidthere.gif)

See also: Schumacher's introduction to Bodhidharma imagery in Japan.