The walls of the small intestines soaks up all of the liquid in it and it carries it to the rectum and after it travels through the rectum it exits through the anus. That is how a waste molecule takes waste out of the body.
Fishes certainly poo in water. When I kept goldfish it was common to see them swimming around with faeces hanging down. I assume the snail and catfish population in the tank ate it up and the...