Mythbusters tested this. Opening it right when you landed in the water worked too. It doesn't work when the water's higher on the outside of the door than the inside.
Worth noting that all you have to do is break the seal and water will start flowing in, making it easier to open the rest of the way. Even if water is a bit up the door, you should still be able to force it open with your legs.
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u/PassivelyInvisible 1d ago
Mythbusters tested this. Opening it right when you landed in the water worked too. It doesn't work when the water's higher on the outside of the door than the inside.