So with how Serene is laid out, the end of the map is directly next to the beginning of the map, only connected by some water and barriers so that players can't just swim directly to the end of the map. However, whenever you get to the final area of the map, you can swim back to the beginning of the map with the new 1.13 swimming mechanics. You can abuse this to hit the checkpoint sign at the beginning to reset your time, and while still in the swimming animation, return to the final area of the map and finish it. This doesn't skip the whole map, as you can only do the new swimming animation in the final area, but you can abuse this to break the map's speedrun time. None of the times on the leaderboard use this. Video showcasing how to abuse this is shown below.