Considering Ukraine has interest to damage Russian defensive positions on the left bank and that the flooding will last only several days, it has motives.Motive and opportunity all say Russia, by applying just the lightest touch of Occam's Razor.
And considering Russia has lied blatantly and egregiously about all manner of things for years – all of which retroactively bleedin' obvious – why on earth would you even begin to think anyone believes anything Russia says these days? Unless they also have ulterior motives for the convenience of officially believing in something Russia claims?
Also considering that Ukraine has been caught blatantly lying multiple times during this conflict and that it apparently started counter-offensive which doesn't seem to go well, everything points to Ukraine as perpetrator.
For Russia, the only outcome is additional problems with defense and with water supply in Crimea.