Boats were set adrift on the River Yare sparking a three-hour rescue drama.
Hemsby Lifeboat crew was called to the scene in the early hours of Monday morning (November 30) after a 17ft cruiser was deliberately untied.
One other boat was also untethered and drifting along the water, but was retrieved by its owner.
Daniel Hurd, lifeboat coxswain, said he was angry that the suspected "ill-conceived" prank had endangered lives and wasted resources.
He said: "There could have been an emergency call come through and crew members were wasting time in Norwich moving boats about.
"It does annoy me. Why would you walk along there and untie the boats?"
Although there were no people on board either of the boats lives had been put at risk, he warned.
The crew had been called at 3.50am.
It was particularly galling given that the independent service had been forced by Covid-19 to cancel its Santa lifeboat tour which last year raised £8,000, he added.
A spokesman for Norfolk Police said the force was aware, but the incident had not been reported as a crime.
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