Water Blast: Poseidon empowers his Basic Attack to deal additional damage at a much greater rate than normal. In addition, his attack range is extended. His attack also does Splash damage which can thin out a large mob very quickly.
Tidal Wave: Poseidon calls forth a massive tidal wave which deals damage in a directed path. At close range, this is the only ability that can reliably destroy a temple with a single blow. Though it can thin a crowd somewhat, Water Blast is better for crowd control.
Kraken (Ultimate): Kraken and multiple tentacles emerge from underneath the ground to deal massive damage to a base. Though troops and turrets will target Kraken, any damage dealt to him is irrelevant. Once he does his allotted damage, he will disappear. His tentacles will do a small amount of additional damage indicated by the unit strength while above ground.