In the script, the scene plays out as follows:
Renault walks inside the hangar, picks up a bottle of Vichy water, and
Renault: Well, Rick, you're not only a sentimentalist, but you've become a patriot.
Rick: Maybe, but it seemed like a good time to start.
Renault: I think perhaps you're right.
As he quickly pours the water into a glass, Renault sees the Vichy
label and quickly DROPS the bottle into a trash basket which he then
He walks over and stands beside Rick. They both watch the plane take off, maintaining their gaze until it disappears into the clouds.
Just before this, Rick has shot Major Strasser dead.
It appears that this action, and his "becoming a patriot" in the eyes of Renault, inspires Renault to do the same.
Vichy was an unoccupied region of France that persecuted Jews and collaborated with the Nazi government.
By binning the entire bottle of water, he is symbolically severing his ties with the corrupt Vichy government and the entire Nazi regime, thus becoming a "patriot" himself.
This is a complete turn from his character earlier in the film, where he clearly looks out for himself, regardless of whether that means helping the Nazis or the French interests.
This turn in character is clearer in the scene afterwards, when he and Rick plan their future lives together - on the same side.