The ship wasn’t sealed.
If you have an empty water bottle with the cap on and pull it down into a pool then the water pressure will compress the air inside, reducing its volume and crushing the bottle. But if you have an empty water bottle with no cap and lower it into a pool it will just fill up with water. Water inside and water outside will have the same pressure and there is no net pressure on the bottle to make it deform. It will not crush.
The same thing happened with the Titanic. It was mostly full of water as it sank so there was nothing trying to crush it.
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