As for the "the deep" character from boys tv series coming up,

getting killed by a fish because he aparently caused the death of much marine life,

 

no.

my idea requires some kilometers as cross (+),
depending on the depth of the water chosen,

to support the 1 kilometer tower above the sea level,

supporting it from the below.
(that is the height in which a person can breathe without difficulty)

and that cross (+) will be used to build a city around it

with interlocking cubes that will not permit water in,

as many compartments will be made.

I will draw it later on. It follows the space escalator principles.

So it is not nearly as big as a space escalator.

None at all. 4/100 the size. 4% of its size maybe.

And it can also be used to preserve marine life, segments of the areas.