That is hardly a simple solution. In most states, water rights weren't purchased from the state. They were granted and tied to property deeds. In some cases those actually predate statehood.

In theory states could seize those water rights through eminent domain (or a similar process) but under the 5th Amendment the state would have to pay fair market value for this. That would be enormously expensive. States would have to raise taxes on everyone to fund that, which would be political suicide.