When conditions are like this, landing a seaplane is tough. Glassy water requires precise technique and skill.
Landing on a lake with no wind is like landing on a mirror because there is no depth perception. The pilot must have a proper rate of descent and use visual reference to shorelines, islands or rocks since he cannot see the surface of the water. Peripheral vision must be acute.
When done properly, it is very difficult to determine the moment of landing. No big clunks like on a jet liner.