Sextants used at sea find altitude angles by measuring between the stars(or other astronomical objects) and the horizon. There is a correction that can be applied to account for wave height as it affects the "eye height." Azimuth can be found by measuring against stars that are low in the sky, but most of the time you do not need this for sea navigation. "Down" is inferred as the shortest path between a star and the horizon. It takes a bit of practice to get a good measurement from a rolling boat, but it is skill anyone can learn.