The wreck of the Edmund Fitzgerald is located at a depth of 530 feet (162 meters) below the surface of Lake Superior. That's an extremely risky technical deep dive. There were probably more people in space than non-saturation diving at this depth.
Nowadays this type of diving would be done using an eCCR (and backup open circuit), where some software on a microcontroller controls the amount of oxygen in a breathing loop. A scrubber (hopefully) removes the CO2. Changing the gas mixture as you go is required to reach these sorts of depths because oxygen becomes toxic at pressure, and gas density itself can cause issues with breathing.
There's a mentioned fun fact that is what reminded me of this:
The Edmund Fitzgerald, the Kursk, and the Lusitania all sank in water shallower than they were long.
"MYSTERIOUS DOOR WHICH JAMES CAMERON BUILT HIS SUB TO REACH AND OPEN. HE WILL NOT SAY WHAT HE FOUND WITHIN."
o_O