Oh, you betcha, garg. My wife went kayaking on a lake north of Lake Tahoe last week. The State Parks Department provided free kayaks, so outside kayaks wouldn't contaminate the water with hyacinths or snails or any number of contaminants.
Those lakes are beautiful, but they're very, very fragile.
Apparently, water hyacinths have also become a major problem on the Upper Nile in recent years, but not in Nero's time.
-Joe-