A British man is feared dead after he was reportedly thrown from a banana boat at the Santa Clara Dam in Portugal on Monday.
Richard Chapelow had been a guest of Jon Hunt, the billionaire founder of the Foxtons estate agency.
Mr Hunt said: "My family and I are devastated by the loss of Richard...it is a time of great sorrow."
The Foreign Office confirmed it was assisting the family of a British man reported missing in Portugal.
Mr Chapelow had visited the watersports area - an hour's drive north of the Algarve region - with a group of 10 other Britons, according to local reports.
Local volunteer fire chief Mario Batista told Portuguese media four people were on a banana boat with another six on a boat towing the inflatable.
One rider disappeared after all four were thrown from the float, he said.
The Hunt family said they were "devastated by the loss of Richard, following a tragic accident at Santa Clara Dam."
"Our thoughts and prayers are wholly with Richard's family, and we ask that both families are given space to grieve such a painful loss.
"We will be working closely with the Portuguese authorities to understand what has happened, and are grateful to the local police and fire services for their efforts."