What secondary themes also enrich Crane's story "The Open Boat"?
Two secondary themes have to do with humanity's relationship to nature. The opening line of Crane's story says:
None of them knew the color of the sky.
This introduces the secondary thematic considerations, relevant to humanity's relationship to nature, of the characteristics and effects of nature along with the autonomous quality of nature. As for the characteristics of nature, nature carries on whether we even know what it is doing or not, whether we "know the color of the sky" or we don't. A contemporary illustration of this is that the Sun may or may not produce sun spots when it is expected to whether we know what it is doing or we don't know what it is doing (right now, we know it is not!).
The effects of nature upon humanity are such that, again, whether we "know the color of the sky," or we don't, the effect will still be produced. In other words, if it's raining, you'll get wet outside, whether you believe or know it's raining or not. By the same token, the power of nature is autonomous. It continues in it's course whether we can "look" or not: if it's raining, it's raining, whether you look out the window or you don't look out the window.
Another early line(s) introduces another secondary theme pertaining to humanity's relationship to nature. The line(s) is:
A seat in this boat was not unlike a seat upon a bucking broncho ... she seemed like a horse
The thematic idea is that "domesticating" nature is a means by which to negotiate nature. Horses that make that (jump) "at a fence outrageously high" have been domesticated to help humanity negotiate and in some cases conquer nature--though every flood, every plague, every famine proves that nature always has the advantage and will not be perfectly negotiated nor ultimately conquered (though we keep tryin'!). In the story, technology (the open boat) is the "domesticated" element that is to help the men negotiate "outrageous" nature. The story shows that this domestication, so to speak, does in fact help the men--though in the end they do need more help.