[DrCMS]

Have you googled "aqueous silylation"?

I got a couple promising hits right at the top

If you read the experimental it is not really doing the reaction in water.  They add a large excess of solvent and silane.

In other words, there are no existing silylation reagents which are water stable, even the  tris(trimethylsilyl)silane "super-silyl" reagents?

It is not about the stability of the silane part of the reagent.  The trisyl part is very stable in water but to get that group on to the SH or OH you need a reactive Si-X bond and they will also react with the OH in water.