I don't care about water=lagoon, I care about natural=coastline. That has to be tagged around the ocean, and the lagoon is part of the ocean. You can see the current situation when you look here: 
https://tools.geofabrik.de/osmi/?view=coastline&lon=-97.45166&lat=21.61045&zoom=10
As you see, the lagoon is not part of the ocean any more. (Instead the island are now ocean...) Only because part of the ocean is also something else doesn't mean that it isn't part of the ocean any more.