Image by NASA/JPL/MSSS
The terrain below was noted to be smoother than that above, and cited in support of a past ocean having been present. I would argue in response that vast amounts of material clearly does flow on the surface in major channels.
It will naturally, in aggregate, flow downhill and end up blanketing the topographically lower parts of the planet. It is the lubricant that is in dispute here, not the overall sedimentary pattern. Head et al argued that the volume of the area below the possible shoreline is within the range of previous estimates of water on Mars.
As those estimates seem to be largely derived from the amount of water calculated to have been required to erode what I think are fresh and still-growing valleys and channels, the case is slightly out of step from my point of view, the ocean being long gone at best. I do not know enough about the series of terraces paralleling the possible shoreline to comment.
Mike Malin and Kenneth Edgett, commenting on some of the same images (MGS MOC Releases MOC2-180 to MOC2-183, 1 October 1999) as Jim Head and company, concluded that the line in question was not a shoreline.
The floor of the supposed northern water-ocean is now a sand and silt sea, with dunes everywhere. Just as the feeder systems demonstrate massive wind-driven transport to the lowlands, these dunes demonstrate massive wind-driven redistribution of material within them. Liquid water is required in neither instance.
If the valleys and channels of Mars are in pristine condition, which they are, the shorelines of the oceans and lakes they fed should be the same. You cannot bury one set of features and not the other.
The obligatory shoreline dunes and sand barrier islands could perhaps have all blown away, mysteriously, but something else could not have, and is also not to be seen, namely wave-cut cliffs, large and small.
There is an atmosphere and there are serious winds. Hence there would have been impressive waves. These, moving rocks and sand as here, would have eroded any solid rock outcrop on the shore to form cliffs of varying heights, and all the meteorite bombardment of millennia would not obliterate all of them.
On Mars much of the surface is littered with rocks of pebble, cobble and boulder size. Water waves, and the inevitable longshore drift and rip currents that go with them, would move these around and heap them up to form cobble and boulder beaches and shingle bars and spits, which will not blow away, no matter how hard an atmosphere huffs and puffs. They too are not there, and as such there probably never were oceans or even large lakes on Mars. If there were, all the expected geologically robust evidence of their presence has been long obliterated.
So the channels that are supposed to have fed them, being far more ephemeral, should have disappeared too, and cannot be used as evidence for their existence.