Very little is told about the history of the planet Tatooine, but many facts indicate that the planet was not always desert (canyons, Dune Seas, sandy plains, salt flats). Weren't there oceans and/or thickets?
Even an article on Wookieepedia gives some tips:
Once the seabed of one of Tatooine's vast oceans, the salt flat became a dry expanse of a hard-packed fusion of clay and sodium-rich sand blown in from the Western Dune Sea.[1]
The fully canon book Ultimate Star Wars describes Tatooine as having once been possessed of life-bearing ocean(s) and a water cycle that included rain.
According to Jawa folklore, the great Tatooine desert known as the Dune Sea was once a true ocean. Ancient fossil-bearing rock and eroded canyons seem to confirm the Jawas' stories, but most of Tatooine's inhabitants still find it hard to believe water ever flowed on the planet's arid, sand-covered surface.
The fact that the change occurred within the scope of 'folklore' would suggest that the shift was recent, geologically speaking, potentially within thousands (or at most tens of thousands) of years.
The fact that Tatooine even has life and a breathable atmosphere would realistically mean that there was surface water at some point. Tatooine's inspiration from the Dune novels, Arrakis, had oceans at one point, but almost all the water ended up sequestered underground within the bodies of larval sandworms and underground reservoirs created by the sandworm burrowing.
In one comic, Dawn of the Jedi, it was described as "lush" with "gleaming cities set by blue seas":
Tatooine, a lush world where twin suns warm fair skies. A thriving world, where the Kumumgah live peacefully in gleaming cities along its blue seas.
