Naniwa Superstones are resonoid and should not be soaked. I'll admit that a small amount of time (like 1-2 min) in water does make them feel and work better but it's still not what the maker wants you to do and they warn against it.

Naniwa Chosera are sintered stones which need a short soak of about 20-30 min, just enough to allow the surface to absorb enough water that it will expel it as lubrication when pressure is applied. Since pressure isn't used while honing razors a longer soak time may be required to soften the surface a bit. It's a fine line between just enough and too much though, some of the Choseras turn to mush with too long a soak.

I think in the case of the original poster's cracked stone I would attribute it to temperature differences between stone and water although that's obviously just a guess.

Dave