Silicone grease is not soluble in water. It is a hydrophobic polymer and is waterproof.

The reason those of you are seeing your silicone grease disappear out of the cush drive (and spread all over the wheel tyre) is not water washout, or for that matter heat (silicone grease is good for 200C) but because of a side effect of its free-sliding linear polymers (which give it its excellent lubricating properties). These slippery low intermolecular interactions also result in silicone greases having very low surface tensions so they 'spread' a lot more readily than mineral-based greases. It is this 'spreading' property of silicone grease that makes it less than ideal when uncontained (by a seal) and exposed to centrifugal forces (like in your hub).

It's a funny compound silicone, unlike any others and because it's often labelled 'grease', we expect it to behave like the mineral ones we're more used to, but it doesn't!

If you're really interested in silicone then have a read of this scientific paper amusingly, but appropriately titled - 'Why Silicones Behave Funny'...

https://www.dowcorning.com/content/publishedlit/01-3078-01.pdf

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