If the NTIs have access to waxes or resins, they could create water-excluding vessels without ceramics or glass.

I’m not sure that it’s very likely that they would have access to them, though. Biogenic waxes and resins are, to my understanding, evolved (in plants) to prevent water loss, which obviously isn’t a problem that exists underwater. Bees use their wax to create containers that honey can’t soak into (if it wasn’t for the honey, bees would probably just make their larva cells out of paper or mud like wasps).

Does any known living thing that lives exclusively underwater produce waxes or resins?