Now all of this is very fine, but consider this: Sickles didn't agonize for weeks over this drawing the way a fine artist might. It was one of four panels he drew for that day's sequence. And he was required to produce six such sequences a week, 52 weeks a year.
Below are two daily strips that put our panel in context. Don't be put off by the moire pattern -- if you click on the image you'll see the strip at proper size and the shading will display correctly.
In the two panels, notice the heavy reliance on negative space. The final image shows a rescue ship pulling close to island, with an amazing economy of line. The ship itself is mainly a black blog with small white circle suggesting a life preserver and an other white space suggesting the outline of the hull. A few small black blots behind it indicate the boat's wake as it moves. A large black blob to the right represents the island, with some white highlights that show depth.