Hi, Diogo!
Thank you for appreciating the little details :)
The black outline was mainly added for readibility. Otherwise everything became noisy and impossible to parse. And at this size the outline is chunky enough so that it looks good. If this was at a higher resolution, it wouldn't work. It's not a shader. This was an experiment in writing a raycaster (software-mode) so the outline is hacked in there as well. If I remember correctly it checked for ceiling-wall-floor changes (eg. wall segment ended and floor began) and also for differences in textures. Objects always rendered their own outline.
Regarding the game itself: there's a gated-off entrance at the other end of the beach that can be broken through. Punching it with your fists is going to take a while, so there's a wooden club nearby (have to take a swim). Once in, you will come across someone who has the required tool :)
But the game is frustraitingly difficult (somewhat by design but also due to jam time limitations) so I apologize in advance.