水面 and 底: the water surface and the bottom below it
Compare sky reflected on water with a stone at a bucket’s bottom, separating depth below the surface from the container’s full height.
The upper surface of water
水面, read すいめん, means the water surface. 水面に空が映っています。 says that the sky is reflected on the water surface.
水面に葉が浮いています。 says that a leaf is floating on the surface. The surface of 水, read みず, differs from a position deep underwater.
The bottom of a container or body of water
底, read そこ, means bottom. バケツの底に石があります。 says that there is a stone at the bottom of the bucket.
池の底が見えます。 says that the bottom of the pond is visible. Visibility alone does not establish that the pond has no water.
Depth measured from the surface
水面から底まで二十センチあります。 gives twenty centimeters from the water surface to the bottom. It does not state the full height measured from the container’s rim.
容器の底の位置は同じですが、水面が上がりました。 says that the container’s bottom stayed in the same position while the water surface rose. Separate surface change from bottom position.
Is the twenty centimeters in 水面から底まで二十センチあります。 the container’s full height?
Show the answer and why
That does not follow. It measures the distance from the water surface to the bottom. Height to the rim needs separate information.