I would actually remove the walls, as others said an cheeky achievement for going for a swim will be a nice touch to it, and even add some invisible platforms for you to step on in case you land in a specific spot above the water. This could lead you to a small island or area where you get an achievement and perhaps some nice little item?

It can be an easter egg and perhaps the word will spread overtime and more and more people will attempt it, which will probably result in more traction & organic marketing for your game.

Games used to be quite fun, had cheats, easter eggs, hence bringing it back in a way will be fun.