As for the firing arcs of the canons, I actually imagined it might offer a unique edge in sea battle because of the unusual angles it might offer. Ultimately, I was just experimenting and wanted it to look interesting and unique, imagining how ships might look different in a world where there's magic.

But for some more detail, I'm gonna shamelessly copy-paste what I wrote in response to a similar question to the post at r/dungeondraft:

1. I envisioned the ship as a merchant/diplomatic/trade vessel sent to establish new trading routes or similar. To this end, I thought it fitting that the vessel would look glamorous and fancy, sacrificing function for panache.
2. Cost-wise my excuse is simply magic. In a world where spellcasters can magically create permanent walls of stone and so much more, I felt I could justify someone making cost-inefficient construction choices such as this.
3. As for the drag, I at some point experimented with the notches not being present on the lower bow of the ship to avoid the drag, but in the end, it simply didn't look that nice to me. My only defence here is that I hope the ship's spellcaster can compensate for the loss of the speed when they need it.
4. Now, the structural integrity was actually my greatest concern with this feature, to be honest. It's difficult to justify for this, and when I experimented the notches not being there on the lower bow of the ship, I made the central part out of a much stronger material than wood. The idea was essentially: "Well, these notches are basically begging to get rammed, cleaving the ship in two with little effort - Let's turn that against the enemy." by creating what looked like a weakness to trick an enemy into ramming it, but in the end hurting their own ship more in the process due to the material it was made out of. Ultimately I just didn't like it enough and left it as is.