NavList:

I would think that good conditions would vary on what kind of vessel you find yourself shooting from.  For a larger merchant ship (>100 Meters) good conditions consist of a sky that shows the bodies and, equally importantly, a crisp horizon.  There have been many times when the sky above is very clear and many stars are visible, only to have a fuzzy horizon that leads to abnormally high errors.

 

On a far lesser scale for large vessels is sea state.  During even fairly heavy seas caused by anti-cyclones (clear skies but heavy wind and large seas) I am able to get good sights from the bridge deck.  The only real issue is that it takes a bit longer.  The way a large ship moves is much slower then a small vessel so that you can use your body as an anti-rolling device to get a fairly decent sight.  The motion is also more regular so it is easier to keep your feet.

 

Jeremy