How can I tell what sea or ocean my user is in?

Hello, my first ever post here.
In my application, I need to be able to work out what sea or ocean my user is in. My user is basically a point at a given long/lat.
I can work out which country my user is in because I have all the ISO3166 boundary relations and I can detect if I am inside a country’s polygon.
But there doesn’t seem to be consistent and comprehensive sea/ocean area relations.
There are about 63 relations where place=sea but this isn’t comprehensive. The 5 oceans seem to be defined by just 5 nodes.
There are lots of ways where natural=coastline but although land is defined as being on the left and water on the right, there is no information in the tags as to what the water is (or the land come to that).
How can I programmatically determine if, when my user moves themselves to say -50 lat and -10 long they are in the North Atlantic Ocean?
In case it is relevant to the solution, this application is for blind users navigating a map of the world. So just looking at a render of the map isn’t sufficient.
Many thanks for any guidance.

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