That is what I was thinking, with the pitching and rolling of a ship in the ocean how do you really get an accurate reading? I mean dead calm, okay. But with the bobbing up and down, I would think difficult.
Land. Take a reading. A lot of what they did was along coasts.
Also, on a ship, take several readings, average them out, and you have a better idea than none at all. An instrument does not have to be perfect to be useful.