In this book, the family takes all sorts of things that they get from nature and puts them to good use. For example, at one point, they kill a porcupine and they end up using its quills for needles and for the tips of arrows, among other things. The shark's skin does not get used for anything so vital (at least not that we are told of).
In Chapter 2, Fritz shoots a shark. In Chapter 3, they come across the dead shark and the father tells them to get some of the skin because it can be used like sandpaper or a file. However, we never see them actually use it for that purpose. Instead, the only time we see them use the skin is when they wrap it around their legs to help them climb trees more easily in Chapter 7.