Three sisters were taken hospital for checks after being rescued by Hunstanton lifeboat crews on Wednesday evening.
The group had been left clinging to a buoy in the Brancaster harbour channel after being cut off by the rising tide.
Two were then swept towards the harbour when they lost their hold, before being picked up. A man managed to swim back to shore.
RNLI spokesman Geoff Needham said: “This incident was the most serious one we have attended for some time.”