He is misinformed or deliberately revising history to fit his narrative. Ukraine’s damming of the canal was done after the annexing of Crimea, and in direct response to it. The other relevant dam, the Kakhovka Dam was built by the Soviets in the 1950s. It was destroyed (after a few failed attempts) by Russian demolition charges, though they continue to deny it.

There were arguments from Ukraine about the continued Russian occupation of their Sevastopol naval base in Crimea, which Ukraine wanted to end. That was the real provocation that Russia could not tolerate, but that is not so sympathetic of a story as saying the “Ukrainians cut off the water to the poor independence-minded Crimeans” /s

Of course, if someone is going to wholesale believe one side or the other I doubt there is any way of convincing them otherwise, but the facts simply do not fit the Russian narrative. If, indeed Ukraine “damming the canal” was such a great provocation that they had to start a war over it, why then did Russia subsequently destroy Kakhovka dam during said war which dried up the canal anyway? Of course they claim they did not, because it is a war crime, but it’s pretty absurd to pretend that Ukraine had the means to do it given the dam and surrounding area was very thoroughly under Russian control at all times.