Organisers were forced to cancel training this week in the river and postponed the men's race on Tuesday after the Seine was found to be too dirty for athletes following rainstorms last week.
But despite heavy rain overnight, which can stir up pollution, organisers early on Wednesday said that the water levels had been "assessed as compliant" for the women's event and the men's immediately afterwards to go ahead.
The women triathletes dived into the Seine at the historic Alexandre III bridge in the heart of the City of Light before a cycling stage around Paris and then a running stage.
Big crowds lined the route despite the damp conditions.
Later, French swimming sensation Leon Marchand bids for two more Olympic gold medals in the pool.
Marchand had sent the crowd at La Defense Arena pool into a frenzy by winning the men's 400m individual medley on Sunday and can expect another red-hot atmosphere when he races in the 200m butterfly and 200m breaststroke finals.
The 22-year-old swam four times across the heats and semi-finals on Tuesday and hopes to have plenty left in the tank.
"It's a lot for my system, so I'm going to have to sleep well, eat a lot, and I'll be ready," vowed Marchand, one of the poster boys for the Games in the French capital.
US great Katie Ledecky, a seven-time gold medallist, is out to defend her 1500m freestyle title after being relegated to bronze in the 400m behind Ariarne Titmus and Summer McIntosh.
Chinese world record-holder Pan Zhanle is favourite in the men's 100m freestyle final, while 200m freestyle champion Mollie O'Callaghan will be expected to triumph in the women's 100m free.
Hashimoto quest
Japanese gymnast Daiki Hashimoto bids for repeat gold in the men's all-around final, where he is expected to face a ferocious fight from China's Zhang Boheng
If Hashimoto delivers, he would emulate countryman Kohei Uchimura by retaining all-around gold, and keep it in Japanese hands for an unprecedented fourth consecutive Games.
Revenge could be a powerful motivating force for Zhang after China's late collapse in the team final allowed Japan to snatch gold.
In men's basketball, the star-studded United States have their sights set on the quarter-finals as they take on Olympic newcomers South Sudan.
Kevin Durant's return from injury inspired the USA to a rout of Nikola Jokic's Serbia in their opening game, an ideal start for the reigning champions eyeing a fifth straight Olympic crown.
"KD was phenomenal," said LeBron James. "It's almost like he never missed anything."
Novak Djokovic, chasing an elusive Olympic tennis gold, meets Germany's Dominik Koepfer in the third round, while Carlos Alcaraz plays Roman Safiullin.
Alcaraz will then return to the court alongside Rafael Nadal in the quarter-finals of the doubles.
In women's football, title-holders Canada must beat Colombia if they are to advance to the quarter-finals following a six-point deduction over a spying scandal.
Canada, who won their first two group games, have appealed to the Court of Arbitration for Sport over the punishment with a verdict expected on Wednesday.
The USA are seeking to close out the group stage with a third straight win, against Australia.
Though the triathlons were given the go-ahead, there was further disruption, albeit on the other side of the world, as the surfing events in Tahiti were again delayed by unfavourable winds and may not resume until Thursday.
Back in Paris, diving specialists China will seek to continue their dominance in the women's 10-metre synchronised platform, a discipline they have won each time since its inclusion at the 2000 Sydney Olympics.
The first medals in rowing are on offer, in the men's and women's quadruple sculls, while Australia's Jessica Fox has her sights set on another gold in the canoe slalom.
Gold medals are also up for grabs on Wednesday in fencing, judo, shooting, table tennis and BMX.
