Have you heard of the legend of how Athens became Athens?
Pretty much, both Athena and Poseidon wanted to be the city's patron, so both of them tried to give a gift to the city. Athena gave them an olive tree, and Poseidon gave them... salt water.
Also, Poseidon was caught flirting with Medusa, before she was a Gorgon, in Athena's temple. Athena cursed Medusa, turning her into the snake-haired stone-gazed lady we all know and love. Either way, Athena and Poseidon don't like each other.