This one's a toughie. The fishpond that Lettie refers to as her ocean could literally be the ocean of knowledge—either that or it's just one heck of a metaphor. This is the problem with books that have supernatural occurrences: sometimes the lines get a little blurred.
In any other story, having a body of water perpetually mislabeled as something that it's not would be like raising the symbolism flag in the air and then setting it on fire—in other words, it would grab our attention. But in this case, when the boy gets submerged in its briny depths he is gifted with mind-blowing comprehension of everything and anything in the universe. So we'll leave this one up to you: literary device, or just a really cool landscape feature?