Water can exist in the Nether inside fluid containers, though if you try to pump it out (e.g. using a hose pulley) it evaporates. Two ways I have managed to do this:

• Use a water bucket on an item drain.
• Fill a tank with water, put it on a minecart contraption, and carry it to the Nether.

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This issue has been closed since it has been inactive for 3 weeks since it was marked as stale.

Water can exist inside the nether in vanilla cauldrons too. It's not a bug