Throughout the diary that makes up Nausea, Antoine stares out at the ocean and thinks about it as a symbol for how vast and uncaring the universe is. Yup. Antoine is dictating his symbols for us—we don't even have to work for that one.
But sometimes, he has sudden mood swings and thinks the exact opposite, as we find when he:
[…] raises his head and looks at the sea approvingly:— the sea is also a breviary, it speaks of God. Delicate colours, delicate perfumes, souls of spring. (24.292)
Wait—Antoine McMopeyPants said that? Yes indeed. The natural world might not care at all about human happiness and sadness. But it can still be beautiful if Antoine is successful in temporarily forgetting his despair.
Unfortunately, Antoine's moments of happiness tend not to last long. It's only one paragraph after finding the sea beautiful, in fact, before he changes his mind and thinks:
The true sea is cold and black, full of animals; it crawls under this thin green film made to deceive human beings. (24.293)
In other words, the sea is also symbol for how Antoine sees the world more truthfully than other people. Other people just see the surface of the world and find it flashy and pretty. Antoine, though, sees what's under the surface, which is the horror of realizing that the world doesn't care about you or anything you do. Yeesh.