You are only allowed to ask one question so I have had to edit down your question. Please do not ask multiple questions in future.
You might want to think how the presence of the shark ties in with a wider theme in this excellent short story, which is the age-old conflict between man and nature. Crane seems to emphasise the incredibly precarious situation that the men are in by showing how helpless they are in the face of the might of nature. In the context of the times, mankind had experienced the Industrial Revolution and the age of Reason, where more and more about science was being discovered. However, in spite of all the power that mankind had discovered, the men in the "open boat" are still shown to be incredibly powerless in the face of nature.
One way that this is manifested is by the presence of the shark. Note how it is described:
But the thing did not then leave the vicinity of the boat. Ahead or astern, on one side or the other, at intervals long or short, fled the long sparkling streak, and there was to be heard the whirroo of the dark fin. The speed and power of the thing was greatly to be admired. It cut the water like a gigantic and keen projectile.
Note how the narrator focuses on the beauty and strength of the shark. It is shown to be elegant, graceful and all-powerful in its environment, like a "gigantic and keen projectile." The men are forced to realise that this is yet another example of how puny they truly are in the face of nature, where at any moment, a wave or a shark could be the end of them.