OSM is incomplete and inconsistent, fixing it will require a lots of community effort. It's not reasonable to think it will be done anytime soon. Also, even if it was complete and clean tomorrow, as a map source it still lacks data like altitude above sea level that's needed for 3D reconstruction but not necessary for maps. And given the inconsistencies in the existing data, it would not be something we could rely upon. Fixing all the mess in the data is what takes most of the time spent on the importer by far.
During the OSM river import we are querying our elevation data to ensure 2D features match with the terrain.
If all seas, lakes and rivers were defined in OSM, we could use that to build the water level layer instead of using satellite reflectance data, but we'd still have to look up into the elevation data to determine the water levels.
We prefer more reliable data like satellite imagery that contains the water masks, but these are in lower resolution than proper OSM data and suffer from aliasing and filtering. One of the possibilities is to combine the two into a higher detail water mask map, so that we'd have ~30m resolution water bodies for the whole world, and finer shapes where they are available in OSM. However, this also brings in all the problems of OSM - it's not easy to determine algorithmically which OSM entities are erroneous and would actually introduce artifacts, and which ones are a refinement and can replace the rough raster data.