On page 235, it states:"He heard the hounds. Then he leaped far out into the sea."
A beast at bay...
"I'm still a beast at bay- Get ready General Zaroff" pg 235
"Splendid! One of us is to furnish a repast for the house. The other will sleep in this very excellent bed. On guard Rainsford..." pg 235
When Zaroff returns with dogs to defend him, Rainsford has to make a decision of whether to jump off the cliff into the ocean, or he can let the dogs attack him. At this point, he feels very helpless or futile. He decides to jump off the 20 ft cliff into the water.
Once Zaroff sees Rainsford jump, he assumes he died. To his surprise, Rainsford actually swims back to the chateau. Rainsford sneaks into the chateau while Zaroff is not watching, and hides in his bedroom. At the very end, Rainsford comes out and scares Zaroff, and they have a small conversation, and then Rainsford kills Zaroff.