As the marathon swimmers took to the water, once again the River Seine was back in the spotlight at the Paris 2024 Olympic Games.

Moesha Johnson swam a superb race to claim silver behind open water legend, Dutchwoman Sharon van Rouwendaal.

Johnson had led for the majority of the race, but she was pipped by van Rouwendaal, whose incredible surge at the death saw her slingshot past the Aussie to claim a second open water gold of her career.

"It's gonna take a while to sink in," Johnson said.

"We could see the screens as we swam past, so I knew the second last lap that we've broken away into a group of three, so it was just a matter what colour [medal] it was going to be."

As exciting as the racing had been though, all the talk in the build up had been about the water quality that has dogged the organising committee from the moment their bold suggestion to hold these events here was announced.

From a sporting perspective — as well as organisationally — this was an enormous test.

After a far wetter than usual spring and early summer, the Seine is flowing almost four times as quickly as it would normally be.

The first 800m or so of racing was completed by Johnson in a shade under six minutes, so strong was the current.

For the record, Katie Ledecky swam her gold medal-winning 800m in the pool in 8:11.04.

Apples and oranges, though.

The split at 1,500m was 19 minutes, illustrating just how hard the second, up-stream, section of the course was.

"It's just such a tactical race, being in this river," Johnson said. 

"There was a lot of things that you could obviously use to your advantage."

The swimmers hugged the north bank of the Seine so tightly their arms were brushing the vines on the riverside and rubbing against the concrete.

Johnson had taken the race out, but was soon overtaken by 2016 Olympic champion Sharon van Rouwendaal.

However, mid way through the third lap Johnson launched an attack, retaking the lead and dragging van Rouwendaal with her.

Chelsea Gubecka was also in contention, flirting with the medal spots as the race ebbed and flowed, but soon fell out of contention as a lead trio pulled away: Johnson, van Rouwendaal and Ginevra Taddeucci.

Those three were going to fight it out for a medal, with Johnson leading the charge.

But van Rouwendaal, who Johnson described as "the GOAT of open water swimming" used the current around the Pont des Invalides to slingshot her way past, mirroring a move she had made on the opening lap.

The 2016 champion and silver medallist in Tokyo powered away to claim a thrilling victory ahead of Johnson, who held on to take silver ahead of Taddeucci.

"Under that bridge there's these pillars and the current's really strong coming through it," Johnson said.

"There's two options, and I knew whatever option I took, Sharon behind me would have taken the other one and we just had to fight it out and see which route was quicker or who was stronger. 

"That's where she passed me into the gold medal position, but I'm happy with my choice. I committed to it and you know, it's just such a tactical race and there's so many, if buts and maybes that could have happened in that race and to come out with a medal. It's just, it's unbelievable."

Coming into the race, once again all the talk was, quite rightly, about water quality.

A familiarisation swim was cancelled earlier in the week and rumours that triathletes who had competed in the river earlier in the Games were sick were propagating through the village.

However, Belgian triathlete Claire Michel, who was forced to withdraw from the mixed team relay on account of illness, said E. coli was not to blame, and the issue was a virus, according to a post on her instagram account.

After the acclimatisation swim, athletes were enthusiastic about the picturesque course.  

"It is super exciting that this is scenery where we are racing," Felix Auboeck told AP.

"That's probably the best thing we can do for the sport."

He did admit that he was worried about getting ill, but trusted that the authorities would take the right choice, as did Johnson and all the other swimmers speaking in the mixed zone. 

One of those officials, Britta Kamrau, said she was "not afraid" that the swimmers would get sick in an interview with AP, but with an important caveat.

"After all, it's open water swimming," Kamrau said. 

It might be that the question being asked is the wrong one.

Despite all the obstacles, including the far higher than normal water flow and rainfall, Paris has achieved something that has been banned for 100 years — the ability to swim in the Seine.

The 1.4 billion-Euro ($2.3 billion) project to return the river to its former glory represents a huge outlay, but one that is for an enormously noble cause.

"To all those who want to continue saying it's impossible to depollute a river, I tell them, 'Yes it's possible, we did it,'" Paris Mayor Anne Hidalgo said when the river was given the all clear for this race.

One of the key aims of the Rio 2016 Games was to clear up the waterways and reduce the amount of raw sewerage that was pumped into the ocean around the city.

ABC Sport is live blogging every day of the Paris Olympics

Ahead of the Rio Games, an AP investigation found levels of adenovirus and enterovirus comparable with that found in raw sewage at Copacabana Beach, where the open water swimming took place.

Doctor Daniel Becker was more succinct, telling the New York Times that marathon swimmers "will literally be swimming in human crap."

Mario Moscatelli, a biologist and expert in coastal ecosystems, told AP that Rio was a huge missed opportunity.

"We had the Pan-American Games, the Olympic Games, the World Cup, thousands of Olympic promises and environmental legacies that ended up not happening," he said.

Paris is not quite as bad as that.

"Water quality is always something that's on the radar for every location," Johnson said.

"My team prepared for it, the AOC, we had doctors checking in, we had a plan.