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You are assuming that you are conscious when entering the water.I see the warning as more related to likely time to recovery...Will you be spending more than 30 minutes in the water while waiting for rescue. Inland or near shore probably not, but way way out...you may be spending some time waiting and more buoyancy may help with a longer wait.
I agree, it takes about 3-5 minutes underwater before you are a goner, whether it's in a bucket, 4ft of water or 4000ft of water.