Symbolism, Imagery, Allegory
We don't want to get all Hemingway on you, but in A Step From Heaven the sea really is kind of how Hemingway imagines it: your greatest foe and your greatest friend.
How we do know? Because Uhmma tells us so (okay, she actually tells Young Ju), that's how. She reminds Young Ju about how Apa taught Young Ju how to jump into the waves:
He loved the waves, Uhmma says. I remember how worried I was to see you go into the water. But somehow he taught you to be brave that day. You loved the waves after that. Never wanted to come out. (29.47)
Apa teaches Young Ju how to be brave in front of those waves… It's like An Na is practically hitting us over the head with a big banner stating "Braving Sea's Waves = Facing Life's Obstacles" here. But hey—we never said An Na was subtle.