Keep in mind the units. Made an error because I didn't keep track of them.
Heres the solution:
Observe that the rest of the water has no change in its PE. Hence, lets define the base of the ice cube as 0 PE level.
Now the ice is submerged with 18 cm under water. Its (ice cube) centre of mass is at a height of 10cm.
Hence U_i =0.9 ρ Vg\times 10
Finally, the ice melts and occupies exactly the same amount as the volume of ice submerged. Hence, centre of mass of melted water is 9 cm.
U_f= ρ (0.9 V) g \times 9
U_f < U_i
Δ U = - 0.72 \ J