In The Cay by Theodore Taylor, how did Timothy know that the cay didn't have any water by just looking at it?

In The Cay, Timothy and Phillip are stuck on a raft together after the sinking of the ship they were traveling on. Phillip, against conventional wisdom, looks into the sun and becomes temporarily blind. When he finally sees land, Timothy knows that it doesn’t have water because of the size, closeness to sea-level, and lack of people.
The novel says,

"Tis a veree smahl islan', outrageous low."
I repeated, "Are there any people on it?" I thought they could contact my father and then send for help.
Timothy answered honestly, "No, young bahss. No people. People not be libin' on d'islan' dat 'as no wattah." (Chapter 6)

In their conversation, Timothy doesn’t explicitly state how he knows there isn’t water on the island, but it is apparent to him because of his knowledge of islands in general. He sees that the cay is small, which means it's unlikely to have any places for freshwater to collect. It is also low to the ground, meaning it is unlikely to have any running rivers or streams. Finally, it is uninhabited, which points to it being uninhabitable—it is likely there would be signs of people or animals if the island were able to support life. Using all three ideas, he can deduce that it doesn’t have a source of fresh water.