If you put a cup in water upside down so that the air is trapped, there is more pressure, from the water, on the outside of the cup than the inside of the cup. If you place a cup in the water so that it is full and basically just floating around, there is equal pressure on the inside and outside of the cup.
If a ship goes down that has leaks, water will fill it up and there will be equal pressure on the inside and outside.
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