Are you sure your water is tainted? I have Natasha fully inflated at this point.
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Perhaps you already cleansed it, but haven't watched Natasha at 4 pm, when the shrinking would occur? It is weird that you don't have the option to taint it either.
Or are you actively in Umah's quest? I don't think that would change that, but maybe? just trying to troubleshoot. (My save has Umah fully transformed.)
*Edit, I just noticed in Natasha's walkthrough it says you have to have 70 corruption to taint it (I thought it was 65, but walkthrough says 70) so did you taint it then lose a point of corruption, or are you miss remembering this playthrough? (Your screenshot shows 69 corruption).
Gain a point of corruption and try again, easiest way is to sell a potion to Lucien.