Poseidon - Aggressive Mid

Level 1 Boots of the Magi
Meditation
Heavenly Agility
Health Pot x 3
Mana Pot x 3

This starter build gives you a lot of sustainability. Use Meditation when you run out of mana to restore it to 75%, and combined with the three mana pots you should be able to continually lay down attacks in your lane and farm up some gold via minions.

Note: Since you won't really be getting any extra mana until you pick up Gem of Isolation, you will need to keep buying 2 or 3 mana and health potions each time you spawn back at base. It's expensive at that moment, but the sustainability lets you pick off more minions and get a lot more gold by mid game.

Your Heavenly Agility is used for fleeing situations when you are low health and need to get a distance between you and the enemy. On the other hand, you can use Heavenly Agility early game when you're lane-ing against someone who has <70% health to sprint forwards, trap them in your Whirlpool, and finish them off in an aggressive move that they will not expect.

Meditation allows you to stay in lane way longer than you would be able to without it. Whenever you're thinking about running away because you're low on mana, check how much longer Meditation has on its timer. Often an enemy will chase you because they see that you have no mana and they think you're vulnerable, only to be shattered by your combo of Whirlpool and Tidal Surge when you use Meditation right before they attack you.

Boots of the Magi - These are an obvious choice, the mana and magical power are a good start and Poseidon relies on speed early game.

Void Stone - This item is deceiving, it says it only gives you +50 magical power, but the Level 3 aura essentially gives you 25 magical penetration as well, this makes taking out minions with your Whirlpool very quick. Also, the magical defense is perfect because your biggest threat early game is other mages.

Gem of Isolation - This is key to trapping enemies in your Whirlpool. because it slows them even more than the ability originally did. Also, at this point you really need some extra mana.

Obsidian Shard - This item makes the difference between poking at an enemy and finishing them off. You will notice a giant increase in power, more so from this item than from any other item in the build. The penetration helps to make sure that you will hurt the other teams tank as well, which can be difficult without it.

Polynomicon - The extra mana is needed, and magical lifesteal gives even more sustainability. Plus that aura can help you get that last shot off before the enemy escapes.

Rod of Tahuti - This is expensive so it's saved for last, but the massive magical damage increase really tops off this build late game and makes you a threat to any player.

First, note that Poseidon's Changing Tides can make a huge difference in the amount of damage your ultimate combo does. Always try to auto-attack at minions when you can to keep your Tide Level meter up.

Poseidon is, in my opinion, all about being aggressive early game. If you continually lay down a Whirlpool whenever the enemy mid player is within range, it will slowly eat away at their health and annoy them because of the slow effect. Use this and Tidal Surge on the minions who conveniently run in a straight line until the enemy is at about 50-70% health. At this point you will have your ultimate Release The Kraken! and can rush in and take them out with one combo. First use Whilrpool to slow them and eat away at their health, then Tidal Surge once the Whirlpool has disappeared or they have escaped it, then finish them off with Release The Kraken!. If they aren't finished they will be very low, so pop your Trident to get that last sliver of health. Sometimes they will try to flee, this is a great time to use Heavenly Agility and catch them.

Poseidon is powerful from a distance, but can be taken down quickly by a strong AD character, so always stay far away from the enemy. Keep dropping Whirlpool to slow them and stop them from getting too close. Push them back with Tidal Surge .

=== FLEEING ===
Poseidon's Tidal Surge pushes the enemy back a bit, this can save you in a quick escape. Each of his abilities makes him accelerate for a certain amount of time, remember that. And his 2 ability Trident makes him accelerate even more, so use that if possible. And since Whirlpool slows (even more so when you have Level 3 Gem of Isolation), whenever you find yourself running away drop one right below you to slow anyone right on your back. With this build, fleeing is absolutely necessary to survive.

As stated above, this is about being aggressive in the beginning. Continue to poke at your enemy and finish them off the second they are low enough on health. Poseidon has very explosive damage in his combo, use it quickly and you can take out most players at 50-70% health.

At this point if you've been aggressive and persistent in your lane you will be a threat to the other team. It's easiest to pick off another god in 1 v 1, so use tactics to pick them off when they are alone. This should be easy in mid. Use strategies like running up the jungle alongside of middle lane and catching the enemy from behind, or hiding at the edge of the jungle until the enemy gets close enough in the lane that you can jump on them. Remember that Release The Kraken! is great for group fights because of it's explosive area damage and stun in the inner aiming circle.

With this build you wont really need the extra mana from the Mana buff, but if it's available it's always helpful.

The one that gives you an incredible advantage is the Speed buff. You'll be untouchable by anyone who tries to chase you.

As an initiator, the Cooldown Reduction buff helps as well, but I've found that sometimes it just makes me waste through my mana too quickly and it outweighs the advantage. That's a personal choice.