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This depends on how much water you put in and if you ran the engine.
If you drained it right away and refilled with oil then there should be no damage.
If you had drained the engine oil and then filled with water and started the engine then it could be damaged, depending on how long you let it run.
If you had put the water in and the engine had oil in it then less damage would occur but again depends on how long it had been run before you realised your mistake.