One of the things my father loves about his house is his pool. He can go for a swim any time he feels like it, and then go right back home afterwards. No fees, no dealing with car keys, or locker rooms, just a happy dip whenever. One of the things he doesn't love about his pool is the maintenance.
Pools require a lot of maintenance. The water must be circulated, or mosquitoes will lay their eggs in it, bacteria grow and prosper, and pond scum grow, stinking up the entire yard. The water requires chemical treatment to ensure that diseased based organisms can't take root. Organic debris like leaves tend to fall in, and must be fished out. My father spends a lot of time on this. Though it will put many pool-boys out of work, I'd like to automate this work. Pools are nice, and I'd like to make them more comfortable and affordable for everyone.
Already on the market are automated machines to sweep the bottoms and sides of the pool. Experiments with automated quality control tests in factories suggest that a netting machine could work by having many cameras looking at the pool, and bringing down one of many nets on a mechanical arm to remove the offending object. A perpetually running pump circulates the water, but before it returns to the pool, it gets run through a chemical probe, which determines the numbers all pool owners need to know: pH and Chlorine levels. It would have a reserve of chemicals so that it could adjust these levels as necessary. It could also email or tweet when these chemicals are in short supply. Lastly, a trip past an ultraviolet light would kill off any bacteria or virus that survived that far. The then clean water is returned to the pool. Ideally, the pump should be underground, where the noise it makes will not be heard.
It may also be wise to have a reserve water tank. When it rains, dad's pool sometimes overflows, which annoys him. When it doesn't rain for a long time, the water level decreases. (Dad's usual solution for this is a hose.) With the reserve tank, when the water reaches a certain level, we can siphon some off into the tank, and return it later when the water level reaches a lower level. The pool would maintain a certain homeostasis. (The water tank would also need to be ultravioleted. We could probably skip on the chlorine, and mosquitoes are unlikely to lay eggs in an underground tank.)