There are two ways that General Zaroff gets people on to his island so that he can hunt them down and kill them.
First, the island is in a particularly dangerous place in the ocean. It is very easy for ships to wreck near the island just because that's the way the ocean is.
Second, though, Zaroff does not just rely on chance. He has a light set up to lure ships to go the wrong way. That way, he can make sure that some ships wreck if he is running low on people to hunt.
Here is the relevant passage:
This island is called Ship Trap," he answered. "Sometimes an angry god of the high seas sends them to me. Sometimes, when Providence is not so kind, I help Providence a bit. Come to the window with me."
Rainsford went to the window and looked out toward the sea.
"Watch! Out there!" exclaimed the general, pointing into the night. Rainsford's eyes saw only blackness, and then, as the general pressed a button, far out to sea Rainsford saw the flash of lights.
The general chuckled. "They indicate a channel," he said, "where there's none; giant rocks with razor edges crouch like a sea monster with wide-open jaws. They can crush a ship as easily as I crush this nut."