The Environment Agency are investigating after the see turned orange at several Cornish beaches yesterday - leaving swimmers complaining of a painful skin rash.
Swimmers had to make a quick dash back to the shore when an "orange bloom" appeared on the sea yesterday afternoon.
Kate Robarts told Cornwall Live that her and five others developed sore patches on their skin after the swim.
"The sea was absolutely clear when we got in at 12.20pm but the bloom reached the shore by mid-afternoon," she said.
"Six of us did suffer patches of skin irritation which looked like jellyfish stings. The sensation was of burning.
"We kept our heads up and got out as soon as we realised we were in the middle of what looked like an orange bloom on the surface."
She said two of the swimmers went to the minor injury unit to get their skin checked after the incident and were advised to shower well.
"We’re all fine, but have slightly sore patches," she added.
Read more of today's top stories
"I don’t think we were in it for long enough to suffer any worse symptoms – I hope."
The discoloration has been spotted at several beaches on the south coast including Porthpean, Duporth and Charlestown.
The Environment Agency says the change in colour could be due to 'red tide' - a harmful algal bloom which can occur worldwide.
The algae releases a harmful toxin which is dangerous for fish and causes the sea to turn a red-brown colour.
A spokesperson for the Environment Agency said: "We will hopefully be able to collect a sample from this morning’s tide if the discolouration is present and take it away for analysis.
"We recommend bathers avoid any discoloured water."
South West Water is also investigating the incident.
