Timothy, as we see as the novel unfolds on the island, is knowledgeable in survival skills. While he does not come out and say directly why he went to the island, he knew that survival for any longer period of time was not going to be possible on the raft no matter what he knew how to do. He also saw how miserable Phillip was on the raft and knew that he needed to be a safer environment. Timothy, weighing his options, knew that survival would be more likely on an island with no drinking water. Considering the region they were in as well, it's safe to assume that Timothy knew that it would rain often enough and he obviously has the skills to capture rain water for drinking. Timothy knew how to survive and he knew he had to take care of Phillip because Phillip was completely and utterly without survival skills when the ship went down.