Remember: it's a river, not the sea, it's a river, not the sea.
It's easy to forget this fact when you're gazing out at the still horizon of water, and even more so when you realize that the river has tides, too.
The ebb and flow of the Rio de la Plata is especially apparent from the vantage point of the swimming pool at Colonia's Radisson Hotel. In the morning, the pool feels like an infinity pool, with the river's waters lapping right up against it. And later in the afternoon, you're suddenly looking down at sand and silt, a dry (or damp) beach where there used to be water.