Because the volume of water is constant, the water surface is the highest when the bottom area is the smallest, that is, the water surface is the highest when the bottom area = length× width =8×6=48.

When the water level is 5, the volume of water = bottom area × height =48×5=240.

When the bottom area is the largest, the water surface is the lowest.

That is, when the bottom area = length× width = 10×8=80, the water surface is the lowest.

At this time, the height of water = volume/bottom area =240÷80=3.

To sum up: the minimum water surface is 3 cm.