Poseidon gives boons that add Knockback to abilities and improve room rewards.
If given Nectar, Poseidon will give you the Conch Shell.
Poseidon's affinity gauge is maxed out at 7 hearts.
Poseidon's favor requires using the Rod of Fishing and successfully fishing up a minimum of 18 fish, and having caught at least 1 fish for that run prior to encountering Poseidon.
Many of Poseidon's boons apply Knockback to your abilities. Knockback causes an ability to push an enemy an additional distance away from Zagreus, in the direction they were attacked from. Knockback effects can help enable environmental damage from Wall Slams and Traps. Knockback effects also combo well with Poseidon's Status Curse, Ruptured, which deals damage to enemies when they move. Area bosses have immunity against Knockback, but will be affected by Rupture damage.