A yacht moored in Holes Bay raised the alarm after a 12ft dinghy with four young people onboard, and a smaller support vessel with two more people were spotted stuck outside Cobbs Quay.
When the RNLI crew arrived, just before 9pm, two volunteers walked ashore to assess the situation before the lifeboat returned to base for the charity’s lighter, inflatable X boat.
An RNLI spokesman explained: “The volunteer crew used the X boat to ferry the first four young people back, by dragging them back in the boat across the mud. The people were so cold so the lifeboat took them immediately ashore at the Lifeboat College slipway.”
Then they returned to assist the second vessel, which was refloated and made its own way back to shore.
Volunteer Helmsman Dave Riley said: “It was one of those unfortunate incidents where you just get caught out by an ebbing tide.
“Working on the mud is exhausting but we are happy to assist as there are areas in the harbour that are treacherous.”
The RNLI was first requested to launch by Portland Coastguard at 8.45pm in fading light.
A spokesman said: “The X boat is an inflatable dingy the crew use in shallow water or on mudflats - it is used generally in areas inaccessible to the lifeboat.
“The lifeboat returned back to the station at 10pm, it then took a further hour to wash the lifeboat and equipment down ready for service.”