Even though there's no stock "Hull Integrity %" reading like there was in stock SHIII you still lose hull integrity after sustaining damage. Just because your crew repairs all the systems that took damage doesn't mean your boat is then back to 100%. Realistically crews could not repair their boats back to that state at sea (obviously). Your maximum dive depth will change after recieving damage.
When you make a deep dive after taking damage pay attention to the lights in your Command Room. Once they start to flicker you know you're reaching your critical depth and you should come back up still they stop.
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