The water comes from the soil. Even if the soil does not contain significant amounts of water ice, the planet has an immense quantity of carbon dioxide in the atmosphere which would be combined with the hyrogen in the rock to produce water.
If the prospect of using such chemical means to produce large amounts of water is deemed to be too impractical, it would not be unreasonable to redirect the orbital path of a comet or water rich asteroid to impact on Mars and thereby resupply Mars with seas of water.
I think moisture in the soil may be the big discovery they’re talking about lately. To those with no real interest it doesn’t seem like much but water in the soil would be earthshaking in science circles.
You really think we have the ability to divert the path of a comet? And even if we could the reason Mars has no surface water is the fact the atmosphere is so thin that the water has either frozen or evaporated at a high rate. New water would evaporate just as fast as the old water did.
To make Mars livable, truly livable, we would have to somehow reinstate a liquid core(molten of course)in the center of the planet thereby re-establishing the magnetic field that would allow the atmosphere to become heavier. With our present technology I don't think this is feasible.