Like so:

Cocaine pollutes the sea due to sewage discharges from humans who use the drug, as well as the illegal laboratories that produce it, study co-author Enrico Mendes Saggioro, an ecotoxicologist at the foundation, told CNN on Tuesday.

And as a side-note: cocaine being dumped at sea was considered as a possibility, but was ruled out:

Previous research suggested that cocaine dumped at sea by traffickers could be responsible for contamination, but that is not the case here, said Mendes Saggioro. “We don’t usually see many bales of coke dumped or lost at sea here, unlike what is reported in Mexico and Florida,” he said.

~~Probably cocaine lost or tossed during sea transport~~

Edit: Sewage discharge and improper waste disposal by producers it seems