Should coastlines create a closed polygon?

Should coastlines create a closed polygon? Please see the result of the overpass API below for the area contained within the square. Only part of the island Charapucu is tagged as coast. The rest seems to be tagged as non-coast waterways. These non-coast waterways connect with the coast and form a closed polygon, but it was my understanding that the coastlines should be able to create a closed polygon by themselves?

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Ce sujet de discussion accompagne la publication sur https://community.openstreetmap.org/t/should-coastlines-create-a-closed-polygon/109239