Solution: Let the frozen water be x cm thick and the unfrozen water be y cm thick.

Then X+Y= 12,

( 1 1+ 1)/ 1 1X = 3.6，

Get X=3.3.

Y= 12-X= 12-3.3=8.7

A: At this time, the water depth under the ice is 8.7 cm.