Symbolism, Imagery, Allegory
George Carlin famously said that swimming is not a sport because it's really just "a way to keep from drowning." And he's actually sort of right—which is why it's pretty symbolic that Dani and Jake are so darned good at swimming.
Dani's not only an amazing swimmer, she also has a mermaid tattoo and a cool nickname to go along with it. But Dani's also sort of caught between two worlds, just like a mermaid (who's both fish and human). She has a sordid past in California but is anxious to make a fresh start in Raynesville. She's also been kind of weighed down by her mom's drug problems in the past, but has managed to pull great grades in school and generally excel. You might say Dani's very adept at staying above water. That's why it's so fitting that the girl is such an awesome swimmer—she's really good at keeping herself from drowning.
Jake is no different. Before Amy died, he was the best swimmer on Raynesville's team. He was the lord of the swim lanes and kind of floated above everyone else. It isn't until Amy dies and he gets blamed for it that Jake begins to feel like he's sinking a bit. This is also around the time he quits the swim team. Maybe he just didn't feel like he could keep afloat anymore?
Swimming is also a big focus for the entire town:
Raynesville was a swimming town. There was a football team and a baseball team, but the swim team was the school's claim to fame. Raynesville infants were dropped into one of the public swimming pools before they could crawl. (9.1)
Everyone in Raynesville likes a winner. The only thing Coach Higgins can see is championship gold, and Jake's dad just wants his son to get in the pool and start swimming again. These guys don't seem to care much about Jake and Dani as people; they just want fast young bodies in the water. It's an interesting twist on the symbol of swimming: On one hand, it's connected to Dani and Jake excelling—but on the other, at least when it comes to the literal pool (instead of, say, the pool of life), swimming is a way in which Jake and Dani aren't fully appreciated for who they are as people and instead only valued for their talent.