1. The Badeschiff, in Berlin, is a public pool on a barge in the River Spree, overlooking the city. During the winter, the pools are covered so swimming can continue year-round. Image from Arsema. Entry is only 4 euros.
2. The Poças do Gomes, a natural rock pool in Madeira, Portugal, is open to the public for a small fee.
3. The Gellert Baths, in Budapest, dates to 1912 and features thermal baths and a swimming pool. Image from OpenTravel.
4. The Crater at the Homestead Resort in Utah is a geothermal spring, located in a crater in a limestone cave, that stays at a constant 90-96 degrees year round. Swim for $11 during the week and $16 on weekends. Scuba diving is also available (the pool is about 70 feet deep).
5. The Icebergs, in Sydney, Australia, is a public pool located right next to the ocean. So close, in fact, that waves often break over the saltwater pool, which means that the water is often quite cold (hence the name). Entry is only 5 AUD. Image from Design Crave.
6. Twice a year, in September and December, the water level at Victoria Falls, on the border of Zambia and Zimbabwe, is low enough to reveal a natural swimming hole, called the Devil's Pool, at the top of the waterfall. Brave souls can swim to the very edge of the pool, where a submerged lip of rock is all that separates you from a 360-foot drop. Image from the Huffington Post.
7. The Grotta della Poesia (Grotto of Poetry), Roca Vecchia, Italy. Image from Conde Nast Traveler.
8. At Johnson's Shut-Ins, a Missouri state park two hours from St. Louis, water cascades over jagged rocks, forming hundreds of tiny natural swimming holes. Photo from About.com St. Louis.
(Images: as linked above)