Aristotle was aware of water density, and he was aware that sea water contained salt. He often comments on the reasons why he and his predecessors thought this was the case.

Book II of the Meteorology contains a large discussion of this, which is initiated by the first sentence of Book II: "Let us explain the nature of the sea and the reason why such a large mass of water is salt and the way in which it originally came to be."

As for the density specifically, you'll find the theory articulated in meticulous detail in the Physics.

For a contemporary scholarly explanation, see: Lang, H.S. (2007). The Order of Nature in Aristotle's Physics: Place and the Elements. Cambridge University Press.

For an articulation by a contemporary physicist, see: Rovelli, Carlo (2015). "Aristotle's Physics: A Physicist's Look". Journal of the American Philosophical Association.

But perhaps we can give a sufficient gloss on the theory here on Reddit.

We need to understand Aristotle's theory of projectile motion. Aristotle believes that things move through space -- but that space isn't empty space. Several of Aristotle's predecessors (e.g., Democritus and Leucippus) believed that things move through empty space, which they call void; several of his successors do, too, such as Epicurus.

But Aristotle and some others, including Plato, believe that things move in a crowded world. We move through a kind of plenum.

But one major Aristotelian innovation is the theory of projection motion. That things in motion move through a medium which acts on and changes the motions of the things that are moving.

Specifically, he argues that that the speed of an object's motion is proportional to the force being applied and inversely proportional to the density of the medium. (If we were talking about things falling, then instead of the force being applied, think in terms of the weight of the object.)

This allows Aristotle to not just perceive the density of water but also to measure it. The less dense a fluid is, the faster something can move through it.

And indeed, in Meteorology Book II, he specifically comments on the density of water and compares it to the density of sea water.