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 - believed to be 29 and a marketing account manager from London - visited the area, an hour north of the Algarve region, with a group of 10 other Britons, according to local reports.
He is understood to be a friend of Mr Hunt's son, Harry Hunt.
The Santa Clara Dam, one of the largest man-made lakes in Portugal, is used for both watersports and fishing.
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 added.
Mr Batista said the lake's depth and poor visibility have made the search difficult.
Divers and firefighters are reportedly resuming the search on Wednesday.
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."