It's a River

Even though all these pictures show the water as wide as an ocean, it's actually a river.  Montevideo sits about seventy miles inland from the South Atlantic.  The Rio de la Plata is the widest river in the world, and you can see that in the pictures.  When you get to Punta del Este, the point where the river meets the ocean, you can really see the difference in water color from brown river water to blue ocean.  And yes, the water here is brackish and tidal, too.