Poseidon's Trident is found on the fourth level, the Abyss, near the end of the level. After you pass the Kraken, you will pass through a narrow opening and take a right. A bit further up you will see the orange trident stuck into the wall on the lower left-hand side. See the video below:
One important thing to note is that all special items do not show up until after you play a level 2 or 3 times. Just play each level a few times focusing on taking pictures of sealife before you worry about looking for special items.