In view of the current extension of geodesy into the ocean areas, the concept of the geoid at sea comes into focus. How is it related to that on land and how can we utilize oceanographic data to identify it? That sounds like a question in the wrong direction, since geodesists usually take mean sea level as a basic concept when talking about the geoid concept on land: the elevations on land are measured with respect to mean sea level; the Figure of the Earth is equated with mean sea level and its extension into the land, if we could get rid of the mountains. It seems such a simple graphic concept, considering that the oceans comprise about five sevenths of the Earth’s surface, and thus constitute the major part of the Earth’s Figure. We lived happily with this concept for more than 2000 years — at least since Pythagoras — because there was no confrontation with oceanic data and no requirement for high precision. But now there are both. While it sounds like a dream come true — to get data about the marine part of the geoid, either by satellite altimetry or from oceanography or both — new problems appear.