Landing a seaplane on extremely smooth water--"glassy-water"--is risky because it's almost impossible to tell how high you are. Look down and see the sky!
The standard procedure is to descend at about 100 feet per minute and continue flying until you see spray. The worst thing you can do is to think you can flare and land. That's almost certain to send you swimming.
This glassy-water landing was in a SeaRey flying boat amphibious seaplane.