Why were the people of Willemstad running out of water and vegetables?

From the text:

Soon, of course, we might also run out of fresh water. It rains very little in the Dutch West Indies unless there is a hurricane, and water from the few wells has a heavy salt content. The big tankers from the United States or England always carried fresh water to us in ballast, and then it was distilled again so that we could drink it. But now, all the big tankers were being held up in their ports until the submarines could be chased away.

__________________________________________________________________________________

Toward the end of the week, we began to run out of fresh vegetables because the schooner-men were also afraid. Now, my mother talked constantly about the submarines, the lack of water, and the shortage of food. It almost seemed that she was using the war as an excuse to leave Curagao.