Hi. I noticed you use tags natural=water for sea realtions. I think there should be only place=sea and not natural=water. This could cause some rendering problems.
Which rendering problems ? This allows additional consistantcy checks (the same that occurs on lakes, rivers and estuaries).
I notices for example that these relations forgot many islands detected by their coastlines; but in no case this caused islands to be "flooded" by waters).
Thiks is really natural=water like the rest, not really "places".
The rendering problems are unproven, not seen at all, other wise there would be problems as well for estuaries or lakes. The coastal marshes, rocks, sandbanks, mangroves, bridges over sea, buildings on sea, tidal areas are still rendered with no problem at all (I've checked that in many places, existing renderers for various styles and not just OSM Carto, are not impacted at all and don't need any fix)
The same is true for other smaller objects than "seas", such as natural=water with water=gulf, water=bay... (and not all are "places" with a name but may justexist for other taggings needs, such as access restrictions, dangers, specific protections, areas of scientific or industrial interest, other regulations such as coastal and maritime navigation, pollution, resurgences of non saline waters under sea, flora like herbal fields, aquaculture, historic reasons, specific geological features of the sea bed below these waters...)
We just don't tag and delimit oceans for now (too large and only delimited by coastlines plus place nodes arbitrarily placed), but coastal seas and part of seas are waters: if a relation is creatd for them it should have this water tag as the root needed for other water=* tags.
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