When my father was a boy, he and his brothers swam in the Hudson River... brown on some days, green on others, always polluted from industries up river. He lived to be 86. Similarly, in Rome people often took a swim in the muddy, polluted Tiber River, even though it has been polluted and undrinkable for many centuries. Today, the river always runs green. The saying, "swimming the Tiber" or "crossing the Tiber" at one time became a metaphor for a Protestant converting to Catholicism. Today, it is illegal.... swimming, I mean.
As the rivers nears Tiber Island adjacent to Trastevere, fallen trees, debris and other flotsam don’t escape the pressures of a low-head dam. A swimmer drowning in the Tiber would more than likely be discovered stuck in the dam. Paddling or swimming the river nowadays is a fools game...