As far as I know, V. cholerae secretes a toxin called choleragen into the intestinal lumen which affects the intestinal epithelial cells causing release of Na+ and Cl- ions into the lumen and reducing the lumen's water potential which causes water to flow into the intestinal lumen resulting in diarrhea. How does this benefit V. cholera in any way?
After V.cholerae gets into the human intestine it starts to multiply its numbers, and then becomes virulent after sufficiently expanding its numbers. This virulence drives the diarrhea which in part causes the bacteria to slough off into the intestinal lumen, and then into the external environment again.
So in short it uses the human intestine to increase cell numbers.
See work by Bonnie Bassler for a really fascinating understanding of the complexity of this infection.