This looks to be where they were on the latest CCTV of them the afternoon before they vanished. It actually looks like they are walking back up from the bank towards the regular pavement as opposed to simply walking along the path.
In terms of being in the river or out to see, from the shape of the river it looks most likely they would wash out towards the sea, but given its a tidal river I guess it's possibly the current heading back inland could take them into the port area.
That being said, there are a few breakwater sections which look like they could potentially trap floating bodies if the currents ran more directly out to sea. I assume the authorities are keeping an eye on those.