If the diameter of a cylinder is 40 cm and the radius is 20 cm.

The included angle between the drum and the horizontal line is 45, and the diagonal line is 45. Then the angle between the drum and the inside of the container is 45, the angle between the horizontal plane and the inside of the container is right, and the angle between the drum and the water surface is 45, so the lowest point of contact between the drum and the water surface is in a straight line. The distance from the point where the cylinder touches the water surface to the inside of the container is a container radius of 20 cm. In other words, the height without water is 20cm, so the depth of water in the container should be at least 35cm.