You do well to focus on the setting as a key component of this great and exciting short story. You need to remember that in a sense, Ship-Trap Island, as Zaroff has called his abode, is created by Zaroff as an environment that will lure sailors to it so they will be shipwrecked and then can be used as prey by him for his own enjoyment and satisfaction.
For example, Zaroff has planted lights to draw ships to the island. Note what he says about them:
"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."
So, if you think about the setting, it has deliberately been created to be deceptive. It is very dangerous and misleading as it draws sailors in to have their boats crushed. These are the key aspects about the island as it is described by Zaroff - you can go back and read the story now and find more descriptions you can use. Good luck!