For sea-going vessels, Ghosts of Saltmarsh says that (186):
A ship component is destroyed and becomes unusable when it drops to 0 hit points.
So, for example, if the 'movement component' was reduced to 0 hp, the ship would be unable to move. Further (186):
A ship is wrecked if its hull is destroyed.
Of infernal war machines, Baldur's Gate: Descent into Avernus says that (216):
When an infernal war machine drops to 0 hit points, it ceases to function and is damaged beyond repair, and any souls trapped in the vehicle's furnace are released to the afterlife
However, I have been unable to find any similar statement of consequences for spelljamming vessels while looking through the Astral Adventurer's Guide for Spelljammer: Adventure in Space. I can see that Spelljamming vessels have hp and damage thresholds, and I can see that they can be damaged by PC's, by ship-to-ship weapons, and by Crashing. Various means are given to repair them. However, I am not finding any reference to what the general consequences are of a vessel reaching 0 hp.
Is such a ship unable to move? Does it stop moving immediately, or continue in motion on its last speed and course? Does it lose structural integrity and start to come apart? Can it be repaired from a state of 0hp, or is it permanently destroyed? What happens if it is at 0hp and continues to take damage?
There are some clues in a few places in the text, but it is unclear whether these are narrative flourishes or rules (and if they are rules, whether they are general to all spelljamming ships). For example, the Turtle Ship says:
If the ship takes damage while on or under the water, interior hatches can be sealed to prevent the ship from flooding completely.
Would taking damage once the Turtle Ship was at 0hp mean that the interior hatches were overwhelmed, such that it would flood and then sink?
The Living Ship says:
This ship’s main distinctive feature is the fully grown treant on the aft deck. The treant has a speed of 0 because its roots are woven into the deck; it and the ship are inseparable. If the ship is reduced to 0 hit points, the treant dies of shock.
Here is a specific consequence of a ship being reduced to 0hp, but it seems to be restricted to this one type of ship.
In the Light of Xaryxis adventure,
at the end of chapter 4 and beginning of chapter 5,
the players might be stranded when their ship is "disabled or destroyed". "Disabled" clearly refers to their spelljamming helm being rendered inoperational so that they cannot control their ship. But how could their ship be "destroyed"?
Another ship is commanded to "demolish what's left" of it
Does this refer to doing damage after a ship has reached 0hp?