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 but almost all the water ended up sequestered underground within the bodies of larval sandworms and underground reservoirs created by the sandworm burrowing.  And yes in one comic, Dawn of the Jedi it was described as "lush" with "gleaming cities set by blue seas".  

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