Chapter 5 Page 28 (Patreon)

People have asked me how he would have ended up there and I think from the beginning I said "he swam" and I wasn't lying. Mostly. I imagine he went through quite a bit of ways to end up where he is but still. But Mack was always sort of an anti-mermaid anyway. "Does he have any magical powers?" Nah. "Does he live longer or have legs or some weird biology?" Naw. "How will he reveal himself to Felix and how will he react??" He just knows. "How did he get to this lake in the mountains?" He swam and crawled and clawed his way there. Pretty mundane.

And yes I actually checked the IRL equivalent of where this comic is placed and yes, it is technically possible to swim to it.