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":

> [![Two yellow suns loom close over a world with white clouds, blue ocean, and green land.][1]][1]

> 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.

  [1]: https://i.sstatic.net/rcZrD.jpg