Let's come up with some other possible titles Stephen Crane could have gone with for this story. We're thinking stuff like "The Boat," or, "The Life Boat," or how about "The Boat That Wasn't Much Bigger than a Bathtub"? Okay, that last one is a little dumb, but "The Life Boat" would've worked pretty well at least.
So why would Crane have decided on "The Open Boat" instead?
Well, let's look at that word, open. What are some synonyms that come to mind? We might say: exposed, vulnerable, susceptible. Or we might go another direction, and say: approachable, available, vacant. Both senses of the word are fitting, since in the story, the four men are certainly vulnerable, stuck on a tiny boat amidst the relentlessly pounding waves. This vulnerability leaves them open to asking questions they've never really considered before. As their journey progresses, their views of the universe are vacated, in a sense, which leaves them able to see things differently than before.
Look how much we can get out of just one word. That Mr. Crane was a pretty clever guy.