Suppose the water level rises and can't cross the iron block.

Water level rise 6x6x 6(24x 6)= 1.5 decimeter.

The iron block is 6m high and the water level is 4m high. It takes 2m to cross the iron block, so the assumption is not true and the water level has not crossed the iron block.

Suppose the water level rises by X.

6x6x(X+4)=24x6X

6X+24=24X

18X=24

X=4/3 decimeter ≈ 1.33 decimeter