You probably know that there isn’t water floating around in lakes on Mars, which makes this a little confusing. Actually, liquid water can’t even exist on Mars’ surface for very long periods of time because of its low atmospheric pressure.
Mars has two polar ice caps, however, which are made mostly of water. If just one of those melted, it would be a sufficient enough quantity of water to cover the entire planet- of course the water wouldn’t last forever due to the atmosphere. It is believed that there are large quantities of water ice trapped in Mars’ thick cryosphere (the areas where liquid is frozen in solid form).
It’s also been very strongly suggested by a large number of evidence that water did, at one point, exist on the planet’s surface in liquid form due to geographical formations and landforms.