When swimming with the current in the early stages of the race, Wiffen threatened to crack the top 10 but dropped away when fighting his way back up the Seine, when he was met with 0.3 metres of current per second.
“A lot of people would say it’s the worst race they’ve ever done, in terms of the currents,” he added.
“So for my first one to be the hardest and I came 18th, I’m very happy.”
The marathon caps a remarkable Games for Wiffen, who added Olympic gold and bronze to his two World Championship golds from February in Doha.
He joked his immediate thoughts were about getting a massage after a gruelling swim, and his achievements would sink in when he returned home.
Wiffen added he wanted to carry the flag at the closing ceremony, something he feels would be a “class” experience.
“I’m Olympic champion, Olympic bronze medallist and placed 18th in the open water,” he said. “I couldn’t have asked for anything more.
“I’m the only double medallist [for Team Ireland], so it’s kind of cool. I’ll maybe put it out there that I’m hopefully carrying the flag.
“We don’t know yet but that’s what I want.”