Just a pool, disguised as a pond, with a trampoline instead of a diving board
Holy fuck! I wrote a paper about these kinds of pools several years ago for a class when they were just prototypes. These pools have a natural filtration system that run based on the plants that are in the pool that give the water nutrients that allow it to not only be crystal clear, but you are also able to drink the water because it becomes so clean. And the best part is that once the initial filtration system is installed and calibrated, it maintains itself and eliminates the need for chlorine or constant maintenance like salt water pools.