According to wiki [1] landuse=reservoir and natural=water+water=reservoir are equivalent, but the latter (and preferred) form is missing in default style. Another thing to comment is that all water bodies are treated the same in default style, despite they can vary from a few square meters (ponds) to thousands hectares. I've been playing a bit with area_size() and found some values that avoid too small water polygons at certain zoom levels in MapSource (it may require some more test though). Below is current water_polygons file in my style. Do you think it's worth including such rules in default style? landuse=basin|landuse=reservoir & area_size()>125000 [0x3f resolution 18] landuse=basin|landuse=reservoir & area_size()>35000 [0x3f resolution 20] landuse=basin|landuse=reservoir [0x3f resolution 22] natural=bay [0x3d resolution 18] natural=glacier [0x4d resolution 18] natural=marsh [0x51 resolution 20] natural=mud [0x51 resolution 20] natural=wetland [0x51 resolution 20] natural=water & water=reservoir & area_size()>125000 [0x3f resolution 18] natural=water & water=reservoir & area_size()>35000 [0x3f resolution 20] natural=water & water=reservoir [0x3f resolution 22] natural=water & area_size()>125000 [0x3c resolution 18] natural=water & area_size()>35000 [0x3c resolution 20] natural=water [0x3c resolution 22] natural=waterfall | waterway=waterfall [0x47 resolution 21] natural=sea [0x32 resolution 10] waterway=riverbank [0x46 resolution 20] [1] http://wiki.openstreetmap.org/wiki/Key:water