Kristian:

I'll add my two cents worth.

It depends on exactly how the sub hull failed during its descent to the bottom. The crews' demise will not be the same for all boats.

But, in general....
Taking a WWII U-boat (or any submarine from WWII) on a one way trip to the bottom......

First the hull begins to creak and groan as the hull comrpesses and distorts under the extreme pressure. A floor or other 'flat' surface might bend up or down slightly, indicating that the hull has compressed inward (such things have been noted by subs that went too deep but lived to tell about it). As the sub went too deep, chances are that the coup de grace will be a major leak from one of the hundreds of fittings through the hull, and not the hull itself collapsing. A hatch or major valve (like the engine induction valve, which was up to several feet in diameter on some subs) will unseat due to hull distortion, an induction pipe or ballast tank will rupture a major line, or perhaps the torpedo tube hatch will burst from its hinges. Fatal flooding will ensue. As the water rushes in and the sub grows so heavy it falls like a rock to the depths below, the crew will realize that this is a one way trip and that they will shortly die. This could be slow, but not agonizingly so - a major leak such as the torpedo hatch could send enough water to fill the U-boat in seconds.

If other portions of the hull were sealed off from the compartment with the leak, it may resist the pressure till the hull was crushed, or it may simply resist till one of its major hull penetrations failed, or perhaps a combination: as the hull crushes like a tin can it would probably unseat virtually every major valve and hatch, allowing the water to rush in and equalize the pressure.
The relatively shallow diving WWII sub hulls would probably not dramatically implode the way deeper diving cold war nuke boats did (aka Thresher and Scorpion). The cold war boats have thicker, stronger hulls, which tend to shatter when overloaded, while the thinner, softer steeled WWII boats will tend to buckle.

Power and light would fail, and the crew would feel the water rush in, surrounded by inky blackness.

They might drown, or the air or water might crush them. Remember that divers take time to acclimate themselves to the pressure - if a wave of incoming water suddenly spiked the pressure to 25 atmospheres (ie: 250 m depth) the crew may find their lungs crushed, with sea level air trapped in their bodies. If the pressure rise was slower, they would perhaps have a chance to equalize the pressure, then drown.

If air was trapped in a bubble, its temperature would rise as it is compressed. I'll have to get out my real gas charts, but off hand I don't think 250m will be deep enough to compress to ignition temperatures. I'll get back to you on that. However, the air would get very, very hot, perhaps scalding those who tried to breath, and crushing those who didn't.

Whereas a cold war boat, prone to a dramatic implosion at thousands of feet of depth that will lead to instant death for all aboard, I theorize that the deaths of a WWII submariner on a fatal one way plunge would probably have been slower.

Regards,

Kurt