Paul Gallihawk, 34, failed to finish the initial swimming part of the race at Leybourne Lakes and was reported missing after he did not arrive to collect his bike for the cycling stage.
The cause of his death remains unclear.
His girlfriend Hayley said: "He was the most amazing and thoughtful partner. I loved him to bits and always will."
"He was incredible in every way and he always put others before himself," she added.
In a statement, the family said: "[Paul] had such a caring nature and will be sadly missed by all the family, extended family and friends.
"Paul was a brilliant son and brother to Nick - he would do anything to help anyone and would always put them first."
His friends also issued a statement: "Many of us have known Paul since we went to school together and we are all completely heartbroken.
"He was a genuine and honest man - he was the funniest person in the world."
The triathlon event was organised by Ocean Lake Triathlon, who said: "We are shocked by what happened."
The triathlon club, based at Leybourne Lake Country Park and Simon Langton School in Canterbury, organises a series of six sprint distance races.
Mr Gallihawk had been missing since a 750m swim at the start of the triathlon.
He had written on his JustGiving page that he was competing to raise money for King's College Hospital in London, which had looked after his father. Since his disappearance was announced, donations on the page have risen to over Â£6,000.
One donor wrote on the site: "Paul, you always had a smile and a laugh about everything, and a great teammate playing for Lenham. An absolute gent and our thoughts are with your loved ones."

The Christmas Day meals will take place in Manchester, Leeds and London after Mr Sissay, who is Chancellor of Manchester University, raised Â£38,000.
The poet, who was in a Wigan children's home from the age of 12 to 18, said he wanted to create happy memories.
A group of volunteers has sourced the venues, transport, food and presents.
The dinners are being provided for care leavers in the three cities who are aged between 18 and 30.
"At its most simple the Christmas dinner offers a memory so next year the person can look back and think I was worth something then," Mr Sissay said.
"Christmas Day actually exposes the inadequacies of a system which treats children as if they are a problem because that day the child stands alone thinking 'I must be a problem, I'm obviously not worth anything because there is nobody here'."
The 48-year-old said his own early Christmas memories were "without family in virtually empty children's homes with staff who didn't want to be there".
"You have this secret room inside yourself and as Christmas approaches you draw back to that room. Each time you hear or feel the nature of family you are reminded of what you just do not have."
Mr Sissay said he believes people continue to feel isolated at Christmas after leaving care.
The first Christmas dinner took place in Manchester in 2013 and was extended to a second city, London, in 2014.
The money has been raised through crowd funding.
Tributes have been paid to a triathlete whose body was found after he disappeared during a race near Maidstone on Saturday.
[NEXT_CONCEPT]
Young people who have spent part of their childhood in care have been invited to special Christmas dinners by Manchester poet Lemn Sissay.