Saw last night on the Science Channel that the water was extremely salty and welled up occasionally due to the interior of Mars still being hot. Of course we won’t really know until we get there...
That is what you';d expect. Our own oceans became salty at the time of the flood and the flood in all likelihood amounted to the oceans of Mars getting dumped onto this planet. That would also be the basic answer to the question of where Mars' former oceans went.