In that link, I just drew 5 diagrams over sketchup.

I am trying to explain to you why it would work Mr Musk.

 

drawing 1 = frame for base battom boulder. The one that goes to the very bottom where it meets the sea floor. This is just a frame. It is metal. Iron. That is why it is colored this way.
You pour concrete/compressed earth with it surrounding it. Binding it together.

drawing 3 = said boulder. the very bottom of the walls. You can see by the thickness of the bottom, if the ground is flat enough, which can be leveled, the water cannot penetrate through the pressure made between the ground and the boulder. the distance in thickness is too much. though that may depend on depth of the water as well.

drawing 2 = another frame for the walls that go on top of the very bottom boulder.

drawing 4 = after the concrete wall is added to the metal frame. The metal need not this be this thick this is just an representation by the way.

drawing 5 = with the pillar. the black line on the pillar represents a track for welding machines. it will follow that track and weld the metal portions protruding from the walls to the pillar and the bottom below. leaving no room for any gaps.

I cannot say this is complete. However, following another interlocking slide, near to the pillar and wall. in between it, from another angle. can be extra objects dropped down to reinforce the layers in that area vertically. as that may be the weakest point.

i left out interlocking designs here to save time, but i've drawn it before.

so basically, the same idea to the sides of the walls, slide down, follow a pre-designed track on the walls, weld between when it contacts each other.

leaving no traces of gaps.

and maybe more can be reinforced with more tracks to the walls as interlocking slides.

more objects to reinforce between the welds.