One degree of error in the vertical for the drone's control system, if it's hovering by blowing air downward at 5 meters per second, would be a ground speed of 87 mm/s (sin(1°)×5m/s) in whichever direction the tilt is. Also without any correction in the propeller speed it would result in a loss of altitude averaging 0.76mm/s (2.7 m/hour, (1 - cos(1°) 5m/s). But that could also be caused by something like a mild downdraft, while the horizontal drift could be caused by an imperceptibly weak breeze.

So I don't really know how this is normally done. If you can set the drone on the ground for a few minutes, you should be able to get a very good reference up-vector, but I don't know how long the MEMS gyros can preserve that up-vector without GNSS once it takes off.

At sea you can probably look at the horizon with a camera unless it's foggy.