Wow. I've recovered .30-06 ball, lots of it, that has remained intact after firing into wood or sand. Same thing with .223. I've never seen them fragment, unless you hit a rock with it. Incidentally, shooting at water is almost like shooting at a rock (at rifle velocities). I'm not surprised that the ball fragments under those conditions.
Soaking wet phone books are a poor man's ballistic gel, BTW. It moves out of the way of a bullet quite nicely, and leaves a well defined wound channel behind. I've used them to test .44, .38, .357, .223, etc. Ball never breaks up. Ever. Hollow point pistol bullets expand nicely in that medium, and even they don't break up (unless I'm using the wrong bullet for the velocity - like bullets designed for 850fps moving at 1,400 - then they come unglued).
Check out the MythBusters episode on water stopping bullets.
Every round, including the .50cal, fragmented immediately upon penetrating the water. That's the main reason none of the rounds penetrated 6' under water. If those same rounds had been going slower, they might not have fragmented and might have penetrated 6' of water intact.
I was specifically speaking about 5.56mm (M193 and M855), which I've never seen fail to fragment in water or ballistic gelatin. Combat surgeons are retrieving fragments of 5.56mm, never whole.
Also, you can't really compare pistol rounds to rifle rounds. Pistol rounds are designed to retain as much of the original mass as possible. Since the target is 12" of penetration, fragmentation would remove the mass necessary to achieve suchy.
Modern "combat" ammunition is designed to fragment, lest they zip in and out without significant damage. Fragmentation is the key for military small arms (after shot placment, of course).