When U-boats goes so fast it could be sort of unmanned drone. Reach coast of USA, launch mini-nukes SLBM nukes or cruise missiles, and return fast. Mini because when attacking from US coast you do not need long range intercontinental missiles, you need less than half of missile fuel. Smaller lighter missiles means you can carry and launch twice the numbers.
If manned U-boat, so fast, whole mission is 3.5hr round trip only, so need small crew, no sleeping space needed, no kitchen chefs needed. Small toilet a coffee snacks enough. Tiny amount of Oxygen for 3.5hr round trip missions.
No facilities like other nuke subs which are for months of living under the sea, thus very simple. Always back to base, after few hours. No engineers on board.
No sonar nor sonar crews, because you move faster than sound, you will also hear nothing.
No need the usual self protections, because no enemy could catch up with you.
You hardly use periscope, you view satellites video live to navigate.
You carry not much of freshwater, less than one day of fuel. So concept is alike micro-sub. Likely not more than 5 crew in total.
It is a Lightning Raid Weapon, you returned to base already while your target is still hot in flame and smoke, enemies you killed are still breathing their last breathe, bleeding. You returned base have tea and take bath already.