How was the underwater scene as well as the scene wherein Ethan Hunt jumps into the water tank shot? Was the tank built as an entire functioning set or was part of it CGI?
Interesting article on it.
The stunt first required Cruise to jump off a 120-foot ledge (the bottom was all CGI, but he really did the jump).
Then, in an underwater set that was filled 20 feet high with water, Cruise had to hold his breath as he acted out the scene.
Basically, he did all that scene underwater. It was mostly practical with some cgi to add in the moving parts and such.
Hollywood Insider also featured an interesting video which details the training Tom Cruise went through to prepare for the scene.
Here's another article that details the scene with some cool pictures and such.