Australian Moesha Johnson has celebrated her Olympic marathon swimming silver, earned the hard way in the brutal polluted currents of the Seine, with a Coca Cola - and a dose of antibiotics.

The Tweed Heads, NSW, swimmer was overjoyed to achieve her medal dream, losing out in Thursday's magnificent 10km duel only to her best friend and training partner, Dutch great Sharon van Rouwendaal.

After a typically rough-and-tumble race in which she picked up scratches from boat plants and bruises from hitting the river wall, she proudly clutched her medal with no concerns about getting ill from her dramatic morning's toil.

"I trust my staff and the medical advice, I trust the (water quality) results were within good boundaries, so I've no doubts," said the 26-year-old.

"Yes, I have had a Coca Cola, but I'll go home and have some antibiotics as well - and that's my plan.

"And the water? I've tasted worse, to be honest.

"But it's not about what the water looks like. I've swum in brown water and it's been really good quality. It's about what the data shows us."

Johnson, who was sixth in the 1500m final in the pool a week earlier, reckoned the race, which came down to a thrilling battle between her, Italian Ginevra Taddeucci and her German-based best pal van Rouwendaal, had been a matchless showcase for open water swimming.

"It was so dramatic and historic. If you don't want conditions like this, you may as well do a 10km in a pool. It's a completely different event. I absolutely enjoyed the challenge."

Early-morning crowds flocked to line the banks around the gilded Pont Alexandre III close to the gold-domed Les Invalides, leaving Johnson astonished that for the first time ever in her open water career, she swam to a chorus of cheers all the way round the six 1.6km rectangular circuits.

She did nearly all the hard yards from the front in the strong currents, with only the fast-finishing van Rouwendaal and Taddeucci able to go with her by the fourth circuit.

"We've all got quite beaten up," she admitted. "One of the boats had plants on it with spikes, so we've all got scratches on us.

"The hardest part was if you moved over a metre to try and pass someone, the current was just that much more dramatic, so we hugged the edge of the wall, and I hit it a few times unintentionally."

The key moment came under the Pont des Invalides, 400m from the finish, where van Rouwendaal switched inside and took advantage of the stronger currents to open a decisive lead which she extended to five seconds by the finish.

Johnson had no regrets as van Rouwendaal regained the title she won in Rio in 2016, clocking two hours three minutes 34.2 seconds to the Aussie's 2:03.39.7, with Taddeucci taking bronze in 2:03.42.8.

Australia's other fancied swimmer Chelsea Gubecka, the 2023 world silver medalist, was 14th in 2:06.17.8..

"Sharon's my best friend, also the GOAT of the sport, so to be right up there with her, sharing that podium it's like more than a dream come true," said Johnson.

"We're both foreigners in our training squad, we're housemates and her family's basically adopted me now just because I've been so far away from home.

"You both put it on the line out there and I don't mind what colour it is, honestly. I fought to the end, put everything out there and so this is like a gold medal to me."