From what I understand, the Navy SEAL pool competency test is the most difficult phase of SEAL training, not because it's physically difficult, but because it's mentally difficult. Fear of drowning is perhaps the most difficult fear to push through mentally. Fear of heights is up there too, but fear of drowning (or being buried alive) is about the worst in my mind.
I've heard that a great athlete could get through all the other phases relying a lot on his ability, but the pool competency test is far beyond being just a great athlete. That's seems like realistic training to me.
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If you're reading this, it's too late.