https://www.cnet.com/news/fortnite-cube-finally-does-something-turns-loot-lake-into-bouncy-lake/

Once the cube landed in the lake, the surrounding water started to bubble. Then, as the cube sunk lower and lower into the lake, the purple color spread out to the edges. Unsure of what would happen, we walked into the now purple water only to find that the entire surface of the lake was now as bouncy as the late, great cube.

So far, it looks like the cube's descent into the lake hasn't impacted any other regions of the map.