Let's put realism aside for just a moment and imagine a world in which boats swim through rivers, skimming out pollutants through natural biological filters (plants). Now let's up the ante, and shape these boats like whales.
Vincent Callebaut can take credit for this idea, a whale-shaped boat garden named Physalia. Powered by solar panels and hydro-turbines (how these sub-aquatic turbines don't slow the boat, I don't really understand), the boat cruises through dirty water, soaking up the bad stuff with onboard plants and purifying water with its titanium dioxide surface.
Whatever, so long as it also eats swimmers on command, I'm happy.