When is seawater not just seawater? All the time! Seawater has bubbles, suspended particles, organisms, and many other things in it. How does this affect sound as it travels through seawater? Let's think about it. Have you ever used a flashlight? Most of the time, the flashlight creates a circle of light on objects that you point it towards, perhaps the ground, a car, or a tree.
However, what happens when it is foggy out? You may have noticed that you can now see the beam of the flashlight.
What is different? When it is foggy out, the air contains many water molecules. When you shine the flashlight in fog, the light is scattered in all directions off the water molecules, making the beam of the flashlight visible. When the light is scattered, it does not travel as far. Therefore on foggy nights the headlights of a car do not project as far as on clear nights.
Sound is a wave just like light. The same thing happens in the ocean to sound waves that happens to the flashlight beam.