To be precise, they are stirring up the mud behind the top gates which are flood prevention gates (and "point" towards the sea). They have been stuck with a two foot gap for ages preventing boats to leave or enter. The gap also means they would not prevent flooding of Lydney during exceptional tidal levels.
It is tempting (and uncharitable) to suspect that the EA are only making these gates operational to prevent flooding and not to enable the mooring payers from using their boats.
The photograph shows the boom with the water jet at the surface. At full extension it is still well clear of the bottom. An onlooker commented that a crane would have been more appropriate allowing the jet to sit right on the bottom and not risk damage to the lock floor as a digger bucket could.
It is an eight day evolution to include clearing the lock itself. The photograph was take towards the end of day 2.