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.