At least two people have died following reports of "people in distress" in the River Cleddau at Haverfordwest.
The paddle boarders are believed to have drowned after being swept away in a sudden flash flood.
A third person was airlifted to hospital in a serious condition as parts of the UK were hit with almost a month’s worth of rain in 48 hours, with more downpours expected today.
At least three people were on boards in the River Cleddau, near the centre of Haverfordwest, Pembrokeshire when they were hit by the flood-water surge.
Local councillor Lyndon Frayling told Wales Online: “I feel so sorry for the people who have been injured and have lost their lives. I have lived here all my life and have seen that river rise quickly and be very powerful.”
The Welsh Ambulance Service said one patient was taken to hospital.
Four helicopters took part in the search operation along with police and firefighters dinghies.
One witness said: “They recovered one body and a young woman was pulled out of the river and wasn’t looking too good.
"They were just below the weir when they were hit by a flash flood which came from nowhere.
“It was raining hard at the time and the river levels are quite high because of bad weather we’ve had this week.”
A spokeswoman for Dyfed Powys Police said earlier today: "A multi-agency search is taking place of the river Cleddau in Haverfordwest following reports of people in distress in the water.
"The Quay Street and surrounding areas have been cordoned off, and we are asking the public to keep clear of the area at this time.
"Ambulance, fire and Coastguard crews, including Coastguard helicopter, are assisting with the search."
The Environment Agency said yesterday there were eight flood warnings, meaning flooding is expected, and 51 alerts in place over the Halloween weekend.
It comes after a week of wet weather in which Honister Pass, Cumbria, had 110mm of rain in 48 hours and two month worth in a week. Some 40 properties were flooded.
Train companies across the UK have warned of potential disruption to some services.
Annie Shuttleworth, forecaster for the Met Office, said: “We’re expecting localised flooding and there will be some travel disruptions. Rain will most likely last a bit longer into Sunday.
