I have one that has been under water for most of the past 20 years and it is still hanging in there, albeit somewhat thinner after all these years. I had a 3000 King that broke after soaking for a couple years. The higher grit stones seem to lose integrity if they soak for a long time. -Doug
I believe it's the clay based stones (the red brick looking ones) from King that are safe to soak - not the others which are likely resinoid. I soaked a King 6k for just a week once and it got all mushy and rubber like and didn't work until it was lapped down about 1/16" from the top. Same thing happened to a Shapton Pro stone that I left in water by mistake too.
clay bound stones, ceramic stones, sintered stones, etc. can be soaked for the most part
resin bonded or magnesia bonded stones should not be soaked for the most part (though some benefit from a few minutes in water prior to sharpening)
there are also other stones which need to be soaked before use, but should not be left in water long term... i.e. choceras, the blue synthetic aoto, some naturals, etc. These should be dried in a cool, well ventilated area, away from direct sunlight
then, there are some stones that are just beastly and can take pretty much anything you throw at them, except maybe a hammer