or just clean out the whole area.

 

If the ocean floor is expected to be of certain material,

one can predict its malleability against the drill.

 

If the drill, as rotating mechanism that is both

horizontal and the vertical,

then, despite being dropped by a lengthy powerline,

 

its coverage can be mapped and programmed like a CNC machine.

 

It will go through everything and flatten the surface out.

 

Repeated usage of sonar, may not be necessary.

 

One can technically go for it, "blindfolded" once the first

imaging is mapped out.

 

Unless the drillbit is weaker than the material of the ocean floor itself,

it will level out the ocean floor quite easily.

 

Visibility is not an issue.

 

At least, with the leveling out the floors.

 

With the cubes, this must be assisted with sensors and sonars,

(proximity to one cube to another, and imaging)

 

and with assistance of underwater robots to slide the cubes-in,

and to trigger rotation of the valve.

 

All is difficult, but feasible. Whether it is worth it to create a underwater city,

no idea.