How does the setting both help and hinder Rainsford in his quest to survive on the island?

The setting of the story is "Ship-Trap Island."  Obviously it's an island.  But it is not a desert island.  It is loaded with trees, brush, and undergrowth.  That is one advantage right there, because it gives Rainsford plenty of places to hide.  Also remember that Rainsford is a very accomplished, world famous hunter.  He knows how to stalk prey and set traps.  The forested island gives him plenty of tools to work with.  He sets traps and diversions for Zaroff, and both of those things help Rainsford survive.  

The island does hinder Rainsford though.  Because it's an island, he has no way of truly escaping.  He can't just keep running and hope to get to civilization.  He's stuck there.  The island is also Zaroff's island.  While it does give Rainsford tools to work with, it is still foreign to him.  Zaroff intimately knows the island, and is able to use that to his advantage while hunting Rainsford.