Coastguard and RNLI crews were called as six swimmers were found in the sea off Plymouth Hoe.
At around 1pm on Friday 19 February, rescue crews were called to the area near The Terrace after a walker noticed one of the swimmers looked to be in trouble.
The passer-by was surprised to see swimmers and took a video of them struggling in the strong waves before uploading it to social media.
She said: "They didn't seem to be [in difficulty], apart from one man struggling against the waves to get out.
"But there were over 10 members of RNLI and coastguard there. The wind was picking up.
"It just seemed unnecessary even for experienced swimmers to be in the water."
Adverse sea conditions can pose a threat to experienced swimmers and the coastguard said entering the water during this time is not advised.
A spokesperson said: "Although the swimmers on this occasion were regular open water swimmers and therefore used to the conditions, we would not recommend entering the water at this time of year if you are not acclimatised to the sea temperature and confident in the conditions that can occur."
They added: "In the second call-out of the day, the team were tasked to Plymouth Hoe to investigate a report of a swimmer who was possibly in difficulty in the area.
"On arrival, the swimmer had already been spoken to by a Border Force colleague on a water craft, and had confirmed that he was not in trouble.
"The team remained on scene for a short while to monitor a further group of swimmers who had entered the water, and safety advice was passed by the Border Force member and ILB crew.
"Plymouth ILB had also been tasked and undertook a search of the immediate location to ensure that the correct person had been located.
"Having satisfied ourselves that no further assistance was required we returned to station."