For me it followed (other than the pattern matching of the sea monster) directly from my backtracking solution to part 1 (versus his solution). One of the cases where doing the first thing that came to mind ("Well, gotta see where all the tiles go to see which are the corners") was the right thing, poor reading comprehension for the win! I specifically missed this line in the problem description which led to his (and many other people's) solutions:
> but the outermost edges won't line up with any other tiles.
> but the outermost edges won't line up with any other tiles.