É simbólico, aparentemente. JKR não diz do que é simbólico, além de denotar simbolismo geral.
JN: You know, what I'm curious about now. What I think is one of the neatest things about the Hogwarts tradition is the entrance ceremony, from the whole riding the boats to the castle to the Sorting ceremony. What kind of traditions is there for graduation and leaving Hogwarts?
JKR: D'you know, John, I'm really glad you asked that, because I felt a huge sadness that I wouldn't write a graduation scene. You know, I really did. I knew-- I mean, I knew from early days that we would never see them graduate. I knew that he would-- well not he, they, all three of them, would not. We would not see them at school during what would've been their final year of education. But I really, during the final book, I kept thinking it would've been-- I felt sad that the book wasn't gonna end with that Feast scene, the graduation scene. But it couldn't, you know, it just couldn't. That's not the way it could've ended. It would've felt far too trite and-- you know, a lot of people felt the Epilogue was too sentimental, I think to have a graduation scene on top of what just happened would've been an absurd bit of anti-climax.
JN: Did you have ideas for what kind of traditions that they would do? Like ride the boats back out of Hogwarts, obviously, I think it's the cutest thing...
JKR: Oh yeah, definitely. No, I think the boats would've been the most poetic and beautiful way to get-- for them to leave. And symbolic in that they-- Harry wouldn't have seen the thestrals again, you know what I mean? It would've been a return to innocence, really. And passage over water is so symbolic, you know, in the history of magic, so, yeah. That would've been great.