Two women and a man died at the scene, while a fourth person is in a critical condition in hospital.
Three paddleboarders have died and one is in a critical condition after a group got into difficulty in a river.
Dyfed-Powys Police said two women and a man died at the scene another woman remained in hospital following the incident on the River Cleddau in Haverfordwest on Saturday morning.
They were part of a group of nine adults who travelled to Pembrokeshire for a paddleboarding excursion.
Company Salty Dog paid tribute to one of the people who died, Paul O’Dwyer.
In a tweet, Salty Dog clothing company said it was saying goodbye with “a heavy heart”, saying Mr O’Dwyer, from Port Talbot, would be missed greatly.
The other five people in the paddleboarding group were rescued and were not hurt.
A member of the public is also believed to have jumped into the river to try and save the paddleboarders, but managed to get out safely, police added.
The exact circumstances surrounding the incident are still being investigated.
Those involved are said to have been a part of the South Wales Paddle Boarders and Salty Dog Co, an organisation based in Port Talbot, according to PA Media.
One woman told the news agency that she had pulled out of the paddle on the morning of the incident as she was concerned about the weather conditions.
Vickie Mckinven, from Milford Haven, said the incident was “absolutely heart-breaking” and she was “good friends” with the group, which “did so much to raise money for charities”.
The incident happened following a Met Office yellow warning for heavy rain that was in place until Saturday morning.
A separate warning for more rain across south Wales on Sunday has since been lifted.
But flood alerts remain in place on rivers in Pembrokeshire, including the eastern and western Cleddau catchment areas, following days of heavy rain.
More than 20 police officers, as well as 30 firefighters from Mid and West Wales Fire and Rescue Service, including specialist water rescue technicians, were involved in the rescue.
Two fire services boats helped with bank and river searches, which also involved Coastguard teams and helicopters.
The Welsh Ambulance Service also sent several ambulances to the scene and ferried the injured woman to nearby Withybush Hospital.
Det Ch Insp Jonathan Rees said his thoughts were with the loved ones of those involved and he thanked emergency service colleagues for their support.
The coroner and the Health and Safety Executive have been informed and the Marine Accident Investigation Branch (MAIB) has sent a team of inspectors to the area to undertake a preliminary assessment.
“The aim of the MAIB’s preliminary assessment is to help with determining the way forward,” said a spokesman.
On Saturday, town councillor Thomas Tudor said it was a “terrible tragedy” and sent his sympathies “to the families who have lost loved ones”.