What depth have we managed to reach into the sea by submarine or other means?

[Marshall]

I have performed repairs on a research submarine owned by the US Navy. The sub was in dry-dock at the time of repairs. It was also undergoing modifications to its hull, as huge steel plates were being added. I was told by Navy officials that it was the "World's deepest diving sub."
I am not able to disclose any additional details, as the Navy considers it to be Classified Information. However, with regard to another person's (very informative and detailed) answer, you may want to "add" some depth to those figures, since another sub has exceeded those figures even prior to the hull-strengthening modifications.