Two or three ideas - even though I´ve no clue about that topic. Further I can´t see if that all around is part of the ship or the environment.
1.) It is barrier what secured the Tirpitz what will layed. Reason - Protection against enemy submarines.
2.) They pump something from the bottom of the water. Maybe Oil? Or drain the water a bit to lower the water line to make it impassable for bigger ships as Battleships etc.?
3.) The absolutely opposite - fill the ground/bottom with split or sand to higher the ground? Same reason as 2.).
Let us go there step-by-step.
They are pumping some liquid into the drum. This cannot be a dangerous liquid as there are no security measures visible, therefore I asume it is plain water that is filled into the drum. They do this to make the drum being boyant in the water. The left side will be used as some kind of plattform that levels the surface once the drum is sunken further after being filled.
The plattfom seems to be about 6m in diameter. There are markings on the top, which read 11, 12, 13, 14, 15, 16.
Allright, now I am lost. Anybody else who would like to take over from here?
Kunduz