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It depends on the type of surgery and that how complex it had been.
You can aspirate the water in post-period because you are under the remnant effect of general anesthesia
and may land up in complications to which neither your surgeon nor your anesthetist want to be confronted with. So the rider was for your absolute safety only.
If you want to clear a further doubt, inform me in a Fresh Query / Follow up Query for further opinion
Wish you a great health!