The men's triathlon was supposed to happen this morning, but overnight a decision was made to delay it. The women's is supposed to happen tonight, and it's probably not going to happen either. The river is too dirty.
The news likes to focus on human waste contamination because it's "funny", but in reality the Seine, like most major rivers of most countries especially where they flow through major population centers, is full of contamination of all kinds, including industrial and agricultural pollution.
Some kind of explicit effort was made before the Olympics to clean the Seine at least enough to make it safe for the triathlon swim. They seemed to have made enough progress in cleaning it that the major of Paris Anne Hidalgo, as a stunt, swam in the river a few days before the games to prove that it was safe - although this had to be done at a secret place and time, as a group of online trolls had openly pledged to do their best to "contaminate" the river once the time and place were announced. Unfortunately, the rain that plagued the opening ceremonies lasted for two days and all that runoff re-contaminated the river. Officials had been hoping for the last few days of sunshine that the contamination would drop again to safe levels, but that has not happened and it's not going to happen by tomorrow, and chances are it's not going to happen before the Olympics are over.
This is probably going to be the major embarrassment of this particular Olympics. The organizers were so set on holding the triathlon in downtown Paris that an alternate location was never arranged, even though there are suitable sites even nearby including like a route right outside Paris where national and regional triathlon events are already regularly held. Officials can't just snap their fingers and move the triathlon to one of these sites because it would take just a massive effort to set up signage, grandstands, the necessary medical and guest facilities AND security on top of all of it. International organizing rules for triathlon DO allow for a "duathlon" in an absolute last resort if some unforeseen disaster makes the swim impossible and there's no time to make alternate arrangements, and officials did at least mention that possibility in a press conference today, but nobody wants that, and it would be basically admitting that the event is ruined.