I loved to introduce this with the classic Egg Demonstration. It catches their interest! And gets the point across.
This can be done in many ways. Depends on how dramatic you are by nature. Do what makes you happy.
I would brag about how I could make an egg float and get all dramatic about it. Lots of razzle-dazzle while putting the egg in the (plain) water.
It would sink. I would look disappointed.
Then I would try another glass or beaker of water. More razzle-dazzle.
The egg would float!
Why? This starts a good guessing game - play Twenty Questions! - they will usually figure out there's something different about the second glass of water that makes it denser.
Once the idea of Salinity has been introduced, talk briefly about salty oceans and then do the Salinity Lab. This lab makes layers of color in test tubes. I would make a big deal about how if they did this lab CAREFULLY, they would get clear layers of color. If they had sloppy technique, they would get MUDDY colors. The room would be full of careful kids, which is always a abeautiful sight to see!
CONSIDER THIS: Introduce the concept of Salinity with the Egg Float Demo. Then move on to the Ranibow Salinity Lab.