To find the length of the diagonal of the square, we first note that the radius of the inscribed circle is 6 centimeters. Since the circle is inscribed in the square, the diameter of the circle equals the length of a side of the square.
The diameter of the circle is: \[ \text{Diameter} = 2 \times \text{Radius} = 2 \times 6 = 12 \text{ cm} \]
Let \( s \) be the side length of the square, so \( s = 12 \) cm.
To find the length of the diagonal \( d \) of a square, we use the formula: \[ d = s\sqrt{2} \]
Substituting \( s \) with 12 cm: \[ d = 12\sqrt{2} \]
Now calculate \( d \): \[ d \approx 12 \times 1.414 \approx 16.97 \text{ cm} \]
Rounding to the nearest tenth: \[ d \approx 17.0 \text{ cm} \]
Thus, the length of the diagonal of the square, rounded to the nearest tenth, is: \[ \boxed{17.0} \]