Just as an epistle on the OP, I was reading Gary Gentile's The Technical Diving Handbook (good book by the way, speaking as a non-technical diver reading it out of interest) and he was cross referring to a Navy study that was published in 1989 about tests on the use of rebreathers below 850 feet.
So although much military stuff is classified, I think we can probably take it as read that the US Navy was diving to those depths (and publishing the results) whilst mullet haircuts were still in fashion, and back when everyone still thought Kevin Costner could act.