Of course, the description of the Apparatus already specifies its AC and hit points. As for what happens when those hit points are depleted: The Apparatus's description says that it is a large object. As explained in the DMG (and basic rules) section on objects:

Down – Legs and tail retract, reducing the apparatus’s speed to 0 and making it unable to benefit from bonuses to speed. To be used as a vehicle, the apparatus requires one pilot. While the apparatus’s hatch is closed, the compartment is airtight and watertight. The compartment holds enough air for 10 hours of breathing, divided by the number of breathing creatures inside. The apparatus of Kwalish does not exist in Ansalon. [4] Publication history [ ] AD&D 1st edition [ ] The apparatus of Kwalish appeared on Geek.com's list of "The most awesomely bizarre items of Dungeons & Dragons". [9]The apparatus of Kwalish returned in the Advanced Dungeons & Dragons 2nd edition Dungeon Master Guide (2e) (1989). The Advanced Dungeons & Dragons 1st edition incarnation was updated for the Encyclopedia Magica Volume One (1994). Though with that said, it does mean that a +3 plate is the same price as +3 ring mail… which seems a bit ridonkulous. Instead, we will say that you need to add the cost of the armor into the price as well.

Rare materials don’t have a price, nor is there a spell we can lean on to help in regards to canceling critical hits - but all is not lost. There is a 6th level Knowledge domain for the Cleric that allows you to turn crits into normal hits. We can lean on that but knowing, if you are a 6th level Cleric, you have access to 3rd level spells - which means that adamantine armor has roughly the same power level, but made permanent… and it only works on the person wearing the armor. The apparatus of Kwalish appeared in the Dungeon Master's Guide (3.0) (2000) and Dungeon Master's Guide (3.5) (2003) as a wondrous item. The System Reference Documents for both editions renamed the item to the apparatus of the crab. [5] [6] Relic: A gnome has double the amount of air when a passenger, or it attuned does not run out of air. The apparatus of Kwalish returned in the Dungeons & Dragons 5th edition Dungeon Master's Guide (5e) (2014). The apparatus was described as a wondrous item of legendary rarity. The Waterdhavian treasure hunter Grinda Garloth owned an apparatus of Kwalish, which she used to scour the bottom of Deepwater Harbor for treasure. [7]Suggested uses for the Apparatus were described in the web article Tactics and Tips: The Forgotten Magic Cache (Part 3). [7] D&D 4th edition [ ] Amulet of Proof Against Detection and Location 300 gp*6 (4+1*2)*1.5 = 2,700 gp Amulet of the Planes