In retrospect, I probably should have paid it more attention when that was the complaint shared between the buddies I threw the game at for playtests. Certainly going by all these comments about it, anyway :) I'd banned myself early on from filling in polygons to maintain the faux vector display art style, but I should have come up with something.
Viewing post in The Princess and the... jam comments
I'm sure you'll come up with something. Maybe there's a certain color, or jagged lines, or something that'll stand out to players as non-traversible. Could also have a mostly safe 'starting area' with mixtures of the two types of boundaries before you get to the water where players can learn what is and isn't a crossable line before they get to the waters edge. Either way its a very minor issue and something that the classic vector games struggled with anyways.