The fishermen who aid Pericles are an example of the common people, whom we have thus far not seen in this play full of kings and princes. They are distinguished from the royals not just by their occupation, but also by their speech--they speak in prose, rather than verse. And as is common throughout Shakespeare, the regular people throw off comments of casual brilliance while going about their daily tasks, showing that even those lower down the social ladder have a very clear sense of the world, even if barely aware of their own knowledge. In this case, when the fishermen talk about the way bigger fish are more powerful than smaller ones in the sea and on land, it takes the unseen Pericles listening in to reveal the insight of the lower classes.
Pericles shows himself to be a real hero in a grand tradition--minutes after clambering to shore from his wrecked boat, he plans to enter a tournament and try for another allegedly beautiful royal offspring's hand, convinced he can't lose.