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.