Timothy has several reasons for abandoning the sea for the "veree smahl islan', outrageous low." First, they have been drifting on the tiny raft for several days and their supplies will soon run out. The waters are shark-infested--"D'sharks always hungry, always waitin'..."--and Timothy is forced to risk his own life to save Phillip when the boy falls overboard. Ahead of them is nothing but "miles o' blue wattah." But the experienced sailor in Timothy senses that they have drifted northward and may be near a group of tiny islands known as the Devil's Mouth (located between Jamaica and Nicaragua) since he believed that
"Dis booby I saw was a blue face, mebbe nestin' out o' Serranilla Bank... dey tell us we veree close to d'shore." (Chapter 5)
Phillip, now blind, wanted to remain on the raft. He believed they would be rescued quicker if they were spotted overhead by plane while on their raft. The cay had "No people. No water. No food. No phones." He wonders if the island might not "be worse" than the raft. But Timothy knew that they would be safer on any patch of land than they would be on the raft.
"... we bettah off on lan'..." His voice was happy. He wanted to be off the sea. (Chapter 6)