When the Sun is high in the sky, seas and lakes appear dark because they are actually very good absorbers of sunlight. It’s only if the Sun is low on the horizon or the surface is rippled that water reflects most of the light striking it.
This is because when a ray of light passes from air into water, its direction of travel alters slightly as it slows down. When sunlight is coming from overhead, this change in direction makes no difference, but when it touches the water at a shallow angle, the change is enough to bounce it straight back off the surface.
Answered by Giles Sparrow