We have  公海上, so I guess this can stay. It's sense 2 of 上(じょう), but perhaps not obvious.

See below:-

Obviously many, many, hits and perhaps simply a + b, but the novice may wonder if "上" is "じょう" or "うえ". In many reference sources where the "上" is added on to the main entry of "道路" only the reading for the main entry is given. This is a case where this entry may really help the novice; and it would not be out of place with the many other examples where you demonstrate the variability of the suffix reading.