The particles out of which Mars was formed had appreciable amounts of water. You might expect a fair amount of water locked up in minerals or at some depth. Of course, there are far less minerals on Mars than on earth as robust life and oxygen contribute to lots of minerals. The rocks from the pictures do indicate plenty of surface water at one time and deep water accompanies that. Volatiles like water are largely gone from the surface. However you might find a biosphere with abundant water at depth. JMHO
The Waters of the Deep are said to have been released on earth. What would be required for Mars’ water to be released from the deep?