When the author states that the ocean is like a patchwork quilt, he is referring to how the ocean is a highly diverse environment. As you already know, a patchwork is an object made up of elements that are completely different from each other, but which are able to harmoniously join together to form the patchwork. The ocean is also like that, as it is formed by several elements that are completely different, but that come together in harmony.
However, the ocean is not infinite, on the contrary, it can have an end, due to pollution and intense human exploitation that degrades the ocean immensely. This is what the author wants to present when he says that the ocean is not limitless.
