[seafarer]

The hull damage indicator is an indication that your hull has suffered structural damage of some kind - holes, plate/rib buckling, whatever. In other words, your hull strength and integrity has been compromised in some way and to some degree. It's a fairly vague indicator though.

A hull that's showing 84% damage is not going to stand up to too much stress though. A near miss by a shell or bomb, or too much water pressure as you discovered by diving too deep. Your poor bashed up hull gave way under the pressure of the water at depth.

Also, hull damage is non-repairable for the most part. At least not by your crew and not while at sea. You need to return to port to get that kind of damage repaired. It is the sim's way of letting you know that the damage is structural and requires the skills and equipment of a good yard crew to fix.

Rule of thumb with hull damage, especially high values like you had, is stay surfaced, or if you must dive, stay as shallow as possible.