The Crimean bridge cost 3.7 billion dollars, do you really think it would have cost that much extra to add in a few pipes to carry water?
Not to mention the following...
https://en.m.wikipedia.org/wiki/North_Crimean_Canal
The canal was blocked in 2014, Crimea got along just fine in the last 5 years before the invasion. I think the Crimea water thing might have been a nice bonus, but it was a problem that had already been solved.
By the logic that the water supply was such a big deal, then it would have made sense for the EU to invade Russia and annexe their oil and gas fields after they used access to oil and gas to try and influence the EU and later cut the EU off. By Russian logic that is.
No. The EU implemented saving measures, and found alternative sources, including alternative energy technologies. In the end they turned Russia's strategy against themselves by cutting Russia off even more than, and way before, Russia cut the EU off. And they will have save hundreds of billions of Euros by mid century from the changes they're making. Tens of billion (EU) a year.
As you said a pipeline would not have been a major challenge. There are some substantial dams within 50km of the Kerch strait on the Russian side. Crimea is also not entirely without water. It has many small to largish lakes and rivers. You'd only need to pipe water in during an emergency drought situation. Also the Black sea is only half as saline as the wider oceans out there. Desalination is not much more challenging than some boreholes out there. Russia has energy to spare for such purposes. So... Spend trillions on a war and risk the wrath of the Western World? Or a few billion on drought mitigation? Crimea only has a population of 2 million. Cape Town has about 2.5X more people living in it. It's about the population of Soweto.
All in all the water supply theory is weak. Crimea isn't even Russian territory. They should be taking it as a given that they only have it temporarily. But, there's something wrong with Putin's head.
It's not even as if Crimea was inaccessible to Russians before 2014. Millions of Russians likely went on holiday there or in the rest of Ukraine during Summer months. Many, many thousands probably lived there most of the time. Russia had a navy base there, which nobody had a problem with for as long as Russia didn't make an absolute pain of itself as with Yanukovych. But it seems Russia just can't be happy with a good thing. No. They have to have the bread buttered on both sides, always it seems. Well... No.