“Sea is the most pure and most polluted water. For fish it is drinkable and life-preserving; for people it is undrinkable and deadly.” (S 61, Cp. R, D 61, K 70)
Heraclitus often relativizes the human experience and human perspective. In numerous places he does this by showing the inadequacy of the human perspective in relation to that of a god, who he envisions as having not a partial perspective, but an objective one. In other places, he relativizes the human experience by pointing to the animal. The god’s eye perspective is neither human nor animal, but encompasses and transcends both.
Here Heraclitus is speaking of the relativity of valuations of the sea. The sea isn’t pure. It is pure and polluted. It isn’t life-preserving. It is life-preserving and deadly. There are varying goods for varying types of beings in nature. By drawing attention to the good of the sea for the animal and the threat of the sea for the human Heraclitus is simultaneously hinting at an objective view of things that understands a holistic unity of opposites.