In fact, this material is born of water and in water, withstands outdoor use (even in cold settings where temperatures can go below zero) very well due to travertine’s nonfreezing nature and ensures adequate non-slipperiness precisely because of the material’s inherent characteristics.
It has always been used for the creation of aquatic environments such as fountains and pools; think, for example, of the many Roman fountains (the famous Trevi Fountain being one of them) and ancient baths.