Waste brine is substantially more dense than regular seawater, so it tends to form a coherent layer that doesn't mix easily: it's why waste brine sinks to the sea-bed and spreads out in a layer.

The waste product is brine. It's an extremely salty sludge that's toxic to marine life. In the same way that fresh water is "just" diluted seawater, but pumping millions of gallons of it into a coastal ecosystem would wipe it out.