So some things to consider:
Keep in mind that U/G really doesn't give you very good options for interacting with the opponent in a lot of scenarios. You have some counters, but unless you really set up with more counter-magic, odds are you aren't going to have that many answers for the opponent.
I like the mid-range feel, but I think you have a bit too much at the top end of the spectrum without much ramping to get you there. Essentially, if someone can drop your mana dorks, they have you dead to rights. If you're not really playing a threat until turn 5-6, you will be done - whether you're going against aggro or just a mid-range deck (nevermind the fact that a control build will love the added turns to set up their long game.
So, how do you tackle these issues? My suggestion would be to focus on the few big hitters you're aiming to use and then drop the rest of them. For instance, you don't really want Aetherling in here, as you really should consider it to be a 7-drop (you always want one more to flicker it). Other creatures that are really out of place here, Master Biomancer and Progenitor Mimic. Master really just pumps creatures that are already big enough to be a threat (that's pretty win-more), and Progenitor is unlikely to generate more than one copy given the removal available in the format - so if that's what you want, why not just run Clone? In essence, the only six-drop creature that you would want to consider running is Prime Speaker Zegana and that's primarily for the card advantage she generates. I would consider sliding at least 2x Polukranos, World Eater into the build to give yourself a solid four-drop threat that has to be answered (which also works rather well with Bow of Nylea if you decide to keep it in there).
As for Swan Song, you can certainly leave it in if it has performed for you, but I can see where Syncopate will be more effective. Additionally, I would consider some Rapid Hybridizations as a spot removal/combat trick to deal with BIG threats (or alternately Curse of the Swine if you have them - note it EXILES, so you can take out the Gods with it too!). Cyclonic Rift is a MUST for you as well.
I would also suggest some added ramp options, whether they come in the form of Voyaging Satyr or something else, as you're going to want to get to six mana as quickly as possible as consistently as possible. If you'd like, I tossed together a quick build that might be more to your liking (http://deckbox.org/sets/534192) or, alternately, there is a version that I saw the other day posted by Frank Lepore (http://magic.tcgplayer.com/db/article.asp?ID=11492) you could investigate, AND, as another alternative, he reviewed a different UG list a while back that is more along the lines of what you're doing (http://magic.tcgplayer.com/db/article.asp?ID=11466) and, lastly, here's a BUG list he tried out a while back (http://magic.tcgplayer.com/db/article.asp?ID=11427).
What I can say, Prognostic Sphinx is an epic threat when paired with Bident of Thassa... I won my FNM last night. I hope some of those lists I've linked you to can help. I think it's an under-appreciated color pairing, but in the same respect, the lack of clear, meaningful removal is a serious problem.