Simbi is a  well known and venerated Loa in Haitian Voodoo. Although an image of Mami Wata is used to portray that spirit, most of the Simbis are considered males, they lives in rivers, lakes and springs. (sweet water), it is generally believed that Simbi usually abducts his/her followers or any random person that they might like, while swimming or boating; bringing them to their paradisaical underwater palace  according to legends. Abductions are usually not forever, after 121 days, victims are sent back with all the knowledge needed to be a Hougan or Mambo. If you steal her golden comb, you will be sent back even faster and will be rich for the rest of your life. The same legend goes, when abducted, if you do the complete opposite of everything asked of you, you will be sent back; rapidly, without any spiritual knowledge of course. 
Most Simbis are sorcerers specializing in dark magic and usually served in secret Voodoo societies. (sanpwèl) 

Plants, poisons and dark magic are usually their area of expertise.
As a snake, Simbi's possession resemble that of Damballah, but in a more violent way. 

Alternatives: Simbi Andezo ( Simbi of Two Waters.) Simbi Makaya, Simbi Anpaka