Symbolism, Imagery, Allegory
Anyone who's seen Finding Nemo knows that the ocean is a really good metaphor for life.
You know it, we know it, and the fishermen in Pericles know it. When we first meet them, they're chatting casually about how all the biggest fish in the sea swim around gobbling up all the smaller ones, which is kind of what people do to each other on land: according to the fishermen, the fish in the sea live "as men do a-land; the great ones eat up the little ones" (2.1.28).
Pericles overhears this and thinks these fisherman are pretty wise, especially since they're able to use the sea as a metaphor for social power and class hierarchy.
Just like life, the ocean can also be a dangerous and stormy place, full of tragedy and suffering. This is especially true for Marina, whose mother goes into labor during a tempest at sea and appears to die. Check out what Marina says years later after being separated from her dad and losing her beloved nurse, Lychorida:
Born in a tempest, when my mother died,
This world to me is like a lasting storm,
Whirring me from my friends. (4.1.17-19)
It doesn't get any clearer than this, ladies and gents. Marina's stormy life reminds us that all human beings are completely vulnerable and often have little or no control over tragic events. Sounds like a major bummer, right? So, what are we supposed to do when catastrophe strikes, and life seems like one big tragedy? According to the play, we've just got to hang in there until the storm passes.
This becomes pretty clear when Pericles hears that his beloved daughter is dead, gets on his ship, and encounters a tempest for, like, the gazillionth time. Gower tells us: "He bears /A tempest, which his mortal vessel tears, / And yet he rides it out" (4.4.29-31).
We can interpret this in a couple of different ways. On the one hand, riding out a terrible storm seems like a metaphor for Pericles's unwavering patience and determination. On the other hand, the dude really doesn't have much choice—he's got to just hope for the best and keep on sailing. Either way, Pericles wouldn't make it to the end of his own play without a little hope and perseverance.