Why does Alice find herself swimming in salt water? How does she offend the mouse she encounters?

Alice slips and falls into a vast body of salt water. It is the pool of tears that she cried when she was a giant. She sees a mouse swimming through the little sea, and tries to talk to him, but she unintentionally offends and frightens the creature by talking about her cat. The mouse can talk. Alice offends him again by bringing up a dog that kills rats, and the mouse seems to be swimming away, but when Alice calls out to him and apologizes, the mouse swims back and tells her to swim to shore with him.