Bathing lakes and naturalized swimming pools are part of Nature-based Solutions, NbS.
They are defined by the International Union for the Conservation of Nature, IUCN, as "actions for the protection, sustainable management and restoration of natural or modified ecosystems that address societal challenges in an effective and adaptive manner, while ensuring human well-being and biodiversity".
"A biological pool is a nature-based solution that addresses bathing water safety and health from an ecosystem approach. It applies natural processes as a basis for keeping the pool water in optimal condition for bathing and reduces the impact of standard treatments, which helps to preserve freshwater biodiversity by restoring aquatic ecosystems. Despite its size, the spread of this model in the hotel and resort business community would have a large short-term impact on water use, pollution and the reduction of carbon emissions linked to ecosystems".
IUCN has published a brochure in which, among other things, swimming ponds are presented as exemplary NbS.
https://www.iucn.org/sites/dev/files/content/documents/towards_nature-based_solutions_in_the_mediterranean.pdf
The European Union is also involved in this area and wants to become a pioneer of NbS (EU Green Deal).