I watched a documentary about USS Kentucky, a missile boat of ours.
As soon as they submerge to a certain depth, they stop the boat and do a stem to stern inspection looking for leaks.
These are the most advanced undersea craft available, and even the US Navy leak checks their boats.
That is respect for the elements for you. I was very impressed.
My old, WW II era diesel boat leaked about 10-15 gallons a minute at periscope depth (60 feet). At test depth (400 feet) the leaks dropped to a few gallons a minute.
The pressure at even 900ft is incomprehensible.
Some of the attack boats go down to 2,000ft.