Why are the sailors nervous?

The sailors are nervous because of the superstitions surrounding Ship-Trap Island. The area and the stories makes them uncomfortable.

"One superstitious sailor can taint the whole ship's company with his fear."

"Maybe. But sometimes I think sailors have an extra sense that tells them when they are in danger. Sometimes I think evil is a tangible thing--with wave lengths, just as sound and light have. An evil place can, so to speak, broadcast vibrations of evil.