I just read page 479 of Clair's "Silent victory" and there it says:
"Another of Fife's replacements was Balao, a new boat commanded by Richard Henry Crane. Balao, built at Portsmouth, had a new superthick pressure hull. Her test depth was 400 feet, in extremis, she might survive at 800 feet."
So I guess in theory at least some boats could go way deeper then 600 feet.
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