Crimea has survived without water for years previously. Russia may have determined that the expense of some aquaducts across the Kerch Strait is less than the expense they may incur from a successful Ukrainian crossing of the river.

Or they may have determined they are likely to lose Crimea anyway and are salting the earth behind them.

This is to say, there are plenty of reasons why they may feel motivated to do this. Furthermore they destroyed another smaller dam in Eastern Ukraine only about 2 weeks ago.