It doesn't strike me as practical. Yes, such a thing could be built, but the tradeoffs would not be attractive. The escape section would have to be self- contained pressure vessel. Adding one large enough to carry the entire crew, or two smaller ones that combined could accommodate the entire crew, would require a substantial amount of additional metal. Some additional systems would be needed as well... and for a space that normally wouldn't get utilized. Subs are very much about packing maximum combat capability into the available space. The crew amenities that are included are there to enable extended operation.

Besides... when things go wrong, they generally do so quickly. You want the crew focused on solving the problem, not on possibly trying to make it to some form of submersible life boat. If you are losing the boat in water deeper than crush depth, it's likely there won't be time to evacuate to a detachable escape vessel. If the boat goes down shallower than crush depth and remains intact on the bottom, the Navy has rescue equipment that attach to the escape hatches and evacuate crew members in batches. Reference U.S.S. Squalus.

https://youtu.be/tRzNr9Q9iOw

Submarines, as with space craft, have space at such a premium, you may as well build a second submarine/hull.

"The Space Shuttle was fitted with ejection seats for the two pilots in the initial "shakedown" flights, but these were removed once the vehicle was deemed operational and carried additional crew members, which could not be provided with escape hatches. Following the 1986 Challenger disaster, all surviving orbiters were fitted to allow for crew evacuation through the main ingress/egress hatch, although only when the Shuttle was in a controlled glide, as the crew would have had to reach the exit from their seats and jump out."
https://en.wikipedia.org/wiki/Launch_escape_system

A total waste of money to appease the public conscience by giving the appearance of a chance for survival in the event of an emergency.

Spacecraft and submarines are inherently dangerous. God bless all those brave enough to venture forth in them.

Another thing about submariners and submarines, there is a a lot of personal stress.

My boy wasn't the same when I saw him after he left the Navy as he was on joining Cerberus. A different bloke, no joy of life left. It took years to bring him round and he's still not right. He lives alone with his son here in town.

Sh1t happens to these guys, and, to me at any rate, if they want to clear out the tubes a bit on shore leave and have a few fights, that's just fine by me, and I would fight the bloke who said otherwise.

Appreciate the input gentlemen. Yes, you would have to have serious stress control in those boats. Personally I don’t think I could handle the thought of being in one for any length of time knowing how close death is.
Kudos to them all.

Mick.