Real footage submerged will be hard to come by.. but here's indeed a video showing one submerged and snorkeling
Guess a simulation will be better, almost impossible to film a real boat submerged considering light and depth of focus
Some sequences from 2:20 on, but not "real", of course:
They call it a "Walter" boat here (and of course the type XXI and XXIII boats were derived from those, but had conventional electric drive), but it is a type XXI, here in a SH 4 mod:
Some nice photos but no film..
https://forums.gunboards.com/showthr...-U-S-Navy-1948
While it performed well submerged and at sea due to its new form, it was hard to dock and manouver surfaced in cramped conditions like harbours; also sabotage and poor construction hampered the performance.