The level of faecal bacteria in the Seine was above the World Aquatics standards of 400 colony-forming units of enterococci per 100 millilitres. The women’s marathon swim competition is set for Thursday, while the men are scheduled to race Friday.
Archived version: https://archive.ph/vCwp1
That would make any potential world record not comparable with previous iterations of the event.
I’m more familiar with running marathon, where there are specific criteria that a course has to meet to be eligible for world record consideration. Along these, course must be generally on roads, and the finish line must be within a certain close distance of the start to mitigate wind effects.
They can easily replicate all conditions indoors