From the outside, an almost simultaneous collapse of the failed section combined with a large massive bunch of bubbles, it would look a lot like those scenes from WW2 submarine movies of depth charges going off underwater, except much bigger.
From the inside, there'd be an millisecond or so of seeing a flicker of everything suddenly moving and then the observer would be dead.
IIRC, the Thresher was torn to bits and there was no major hull section surviving. The Kursk, on the other hand, fell victim to an explosion that ruptured her structural integrity and part of the ship telescoped into another part almost instantly, but in water that was well above her crush depth, so that she wasn't torn to bits and at first glance her wreckage appeared surprisingly intact. One of her compartments actually did hold for a time, with survivors inside, but failed itself in a short while and the unfortunate sailors in her died.