At the top of iD (the OSM web editor) there are Point, Lines and Areas.  You added this swimming pool with a line (also known as a way in OSM jargon) which isn't valid and won't render on the final map.  Lines typically are roads, power lines, fences, and very skinny items.

The correct way to do swimming pools is to use the Area and then search for swimming pool.

Hope this helps.