They have fly speeds, but no (hover) like you'd see on a creature. There is magic holding it aloft when a creature is attuned to the helm and concentrating since the helm is a magic item, and in space it doesn't matter much. But what about when there isn't a creature attuned, and there is gravity? If it falls, what about if the creature is attuned but isn't concentrating?
The book doesn't say.
There's no official statement on what exactly happens to a ship if the pilot* stops concentrating, loses attunement, or even dies at the helm. It doesn't say the ship keeps hovering or falls, nor does it even mention what happens in space in such a situation (that is, whether the ship keeps going or comes to a stop).
The 2nd edition Spelljammer books are actually not much more clear. In those books, a ship can slow to a hover while in an atmosphere, and if the pilot is ever killed, the ship "loses power" and continues to "drift" in a straight line ahead, at least in space. It doesn't mention planets that I can find, but the implication that I'm reading is that an unpowered ship just moves under its momentum in space, and presumably losing power in the atmosphere would mean you fall.
I feel that's a reasonable rule, because if the ship can hover while not being piloted, then it seems like there's not much reason to actually land it properly; you could just "leave the engine running" while you wander off to have an adventure. I would probably rule that you can hover the ship, but that requires an active, concentrating pilot like any other maneuver.
But that's just me. If you want to do something else, do it. The Game Police won't kick down your door for doing it wrong.
*I'm going to use the term 'pilot' for the person sitting in the spelljammer helm because calling the pilot a 'spelljammer' is just really confusing when the ship itself is called that too.