I always struggle laneing Poseidon as Scylla. I was wondering (as hard as it is to believe) if there is a way...
Poseidon is one of the best mid-laners in the game currently, and as such Scylla (who has a much weaker early game) will naturally struggle against him. I've written before about ways that a weaker character can play against a strong one here, and some of the advice reamins the same;
Buy Aegis. A 1-2 second invulnerability will protect you from Poseidon's main damage, his Ultimate.
Early ganks. Ensure that your Jungler knows that the match up isn't in your favour, so he can come and help you more often.
Go to solo lane. If you are really struggling, it may be worth switching lanes with your solo laner for a better match up. This may be a difficult proposition to make, but a good team mate will consider it.
More specifically though;
Scylla is a late game god. If you can play defensively early game, you will eventually become more powerful than Poseidon.
Do not put a skill point in your ability at level 8. Your priorites for levelling abilities should be 4, 2, 1, 3. Scylla's passive gives you bonuses when an ability reaches it's max rank, and if you skip levelling up an ability at level 8 you can instead put a point in your 2 and 4 at level 9, rather than waste a point on 1 or 3.
Hope that helps. In short, play defensive at the start, and aggressive later on, and watch out for his Ultimate.