I think it was mainly about not losing face internally when the separatists would have been defeated. Like, there was a bridge. Water can be piped. The port in Sevastopol was big enough. Etc. And there is no way some mines can repay all the costs just as the invasion of Iraq wouldn't be repaid by Iraqi oil ever.
The North Crimean Canal was shut down in 2014 soon after Russia annexed Crimea.
As a water supply it was a greater than a single pipe .. or even ten. Crimea was running critically low on water after a few years and Russia was keen to restore that otherwise the taking of Crimea rang hollow.
No, you underestimate the scale of ecological catastrophe imposed on the Crimea by Ukrainian regime.
" “After five years without water, the population structure of the villages of the steppe regions of Crimea looks depressing. By the end of 2020, 90% of the remaining population will be unemployed. All the rest will leave the region,” concludes Liev.
...
According to Liev, in the last 50 years, when the irrigation functioned, the peninsula received a new layer of fertile land. However, in the last 5 years, this soil has not been moistened enough and is already degraded.
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As an occupying power, Russia is entirely responsible for the maintenance of the Crimean region according to the Geneva Convention, says Permanent Representative of the President of Ukraine in the Crimea Anton Korynevych. Ukraine’s task is to make the occupation as burdensome for Russia as possible. The water supply to Crimea may be effectively used by Ukraine to put additional pressure on Russia so that it returns Crimea." [0]
Note, that the title of this article published by Ukrainian "Euromaidan Press" is "Ukraine’s water blockade of Crimea should stay, because it’s working". Enough said.
