Nice find! But for the survival purposes a simple improvised plumb-line (a rope with weight) is precise enough. And the measurements are less complicated with it. The longer the rope the higher the precision. In fact, the real horizon line is a necessity for the rocking ship situation only (I doubt the artificial one would work on the ship well). If absolutely necessary, an inexpensive bubble level would give you about 1 minute precision (1/60th of a degree), what's more than enough for visual measurements, which theoretically allows to determine your location with ~1mile precision.