Currently we render a blue background and provide landmass polygons for non-water areas. This makes sense given the sea/landmass distribution on earth, but on second thought the inverted approach makes more sense: We will not provide sea tiles for medium / high level tiles, so the sea/landmass tile distribution is dominated by landmass. Furthermore rendering a white background by default will result in a better visual appearance when panning and new (land) vector tiles have not been loaded into memory yet.
To implement this, the background color in vectorosm.dgml has to be changed to white (or whatever landmass looks like), and the landmass tile creation part in vectorosm-tilecreator needs to be changed to subtract the clipped landmass tile from a full water tile, and change the tagging accordingly. For backwards compatibility we either have to provide both landmass and water background tiles for some time, or (likely better) drop landmass tiles directly and store tiles in dev/ or v2/.