Nick Sloman's risk to improve his placing has cost him a top-10 finish in the Olympic men's marathon swimming event.

The Australian dropped from sixth to 11th in the 10km event on Friday at Pont Alexandre III, won by Hungarian Kristof Rasovszky.

Fellow Australian Kyle Lee finished 13th.

On the fourth of six laps, Sloman shifted position in a bid to move up in the field and he burned too much energy.

Sloman ended up swimming by himself on the last lap as he dropped out of the top 10 at his first Olympics.

"It was tough, really tough. The current was very strong, something I really haven't experienced before," he said.

"It was a learning experience, the first couple of laps. The turning buoys were pretty crucial for me, to make up as many places as I could - it was really difficult.

"It was just survival towards the end."

While frustrated, Sloman was proud to compete in Paris after 11 years of trying to qualify for the Olympics.

He finish five minutes 31 seconds behind Rasovszky, nicknamed the Balaton Shark, who won in one hour 56 minutes 24 seconds after finishing second in Tokyo.

German Oliver Klemet was two seconds behind for silver and fellow Hungarian Oliver Betlehem claimed bronze.

On Thursday, Australian Moesha Johnson was runner-up in the women's 10km.

The men's race ended the Olympic triathlon and marathon swim events, which had been in the spotlight because of water quality concerns.

Sloman said the River Seine was not the worst water he had experienced in competition.