get https://api.searoutes.com/geocoding/v2/sea/
Given a point as a coordinates pair, this endpoint checks if it is on sea or not. If on sea, it returns the coordinate pair {longitude},{latitude} as GeoJSON. If on land, it returns the closest coordinate pair on sea.
The field 'valid' tells whether the given point is on sea or not.
The response also contains information about the point : isSeca if the point is in an ECA zone and isCoastal if the point is close to the coast.