Air pockets and you
The way air works in Spelljammer is roughly like this:
Each object has a mass that attracts things to it, including air. So a sufficiently large object (such as, say, a space ship) attracts a large pocket of air to it. If the ship leaves a planet’s atmosphere, it “drags” the pocket of air out with it. Most space ships are large enough to attract an air pocket able to sustain a crew for days (and in some cases, weeks). However, the ship will have to land on a planet every now and then to “replenish” its air pocket.
The diameter of an air pocket is approximately thrice the size of the object’s cross-section. Thus, a five-foot diameter beholder will have an air pocket fifteen feet in diameter.
If an unfortunate crew member gets separated from his ship, he or she may still survive for a short while—the air pocket attracted by a typical man-sized humanoid will last for a little under an hour. This is modified by other factors, of course, such as the presence of the appropriate ioun stone or the Slow Respiration feat. Rescue, needless to say, is still advisable.