One thing people forget about is that water permeates through "solid" soil as well as just sitting on the surface in the form of lakes, rivers, and oceans. These sub-surface aquifers and water tables still exist today on Mars in the form of permafrost and sub-surface glaciers. The water that exists on the surface is mostly in the form of a small amount of water vapor in the atmosphere and ice in the polar caps. The rest of the water that used to exist on Mars has mostly been lost to space along with a, likely, heavier atmosphere at some point. The lighter planetary mass and lack of a magnetic field means that it's comparatively easy for molecules in the atmosphere to directly evaporate to escape velocity or to get dragged away by the solar wind. The same thing happened with much of the water on Venus, even though the gravity is much stronger there (but the planet is also much hotter).