In principle, the oceans and seas are being mapped as a set of anonymous, conjoined polygons, formed by all the natural=coastlines on the planet – at least when the process for merging them doesn’t break due to a data inconsistency. Theoretically, an algorithm such as the conveniently named flood fill algorithm could be used to associate one logical portion of the world’s sea area with a node lying within it. A data consumer would probably need to perform quite a bit of geometry simplification to make this process more efficient.
With the right parameters, a flooding algorithm could probably identify most of the oceans and the relatively sheltered seas such as the Gulf of Mexico, the Mediterranean, and the South China Sea. Maybe it’s also possible to identify “peninsulas” of water such as the Gulf of California. However, the Sargasso Sea and Southern Ocean cannot be defined by coastlines. A manual approach would be necessary, but maybe a companion dataset such as Natural Earth or a one-off GeoJSON file would be more suitable for these features than something hard-coded in OSM, where there’s a tendency toward hyperlocal precision that would be counterproductive in these cases.