pee before takeoff)
Iirc the factor that limited early nuclear submarines the most from continuous dives was the toilets, they have limited storage for sewage and it turns out that draining those with a greater outside pressure is a pretty difficult task, at least if you want to stay undetected.
Now that they have figured that out the limiting factor is food.
You didn’t explain how they sorted it out so I’m assuming compost toilets or some form of slow torpedo that makes it look like the sub is pooing.