I've got a similar UB mill deck too :) In mine, I put in 2x Consuming Aberration , so as to have an alternate wincon late in games, as well as cheap spells that help me control the tempo like Downpour , Smother , Mana Leak , and Crypt Incursion (i found crypt incursion to be very very useful for gaining a large amount of life to lengthen the game). I also reduced the mind grinds to two, because i found that they are more useful when you've got more mana which is at least the 8th turn or so. :) Since you've got sets of a lot of them, I suggest cut down some to 3x. Up to you which ones. :)
It would make all of the instances you have except Blustersquall 1 cmc through the wording each time a card targets something it costs 1 less to cast. Downpour , Blinding Beam and Feeling of Dread all costs 1cmc due to the wording.
I'm liking this. AEtherize and Downpour both seem a bit slow. With Nykthos 26 lands seems like too many. Maybe 23? If so you might have to cut Arcanis the Omnipotent down to 3, which I would do anyways. Laboratory Maniac seems a bit too cute. You won't often deck yourself. But a 1 of Elixir of Immortality would be better for making sure and that would free up 3 slots. Gitaxian Probe and Serum Visions can help you dig for cards with some upsides. Cryptic Command if you can afford it is very good and should be a 3 or 4 of in this deck. Finally, counterspells are a must have in most blue decks. Mana Leak , Spell Pierce , Remand , and Rune Snag are some of the best options. And with an increased number of instants and sorceries some number of Snapcaster Mage s would be nice. Hope this helps, +1!
Hey my names Scott I play at gaming a lot on Wensdays and I think this looks pretty solid. I would definitely play with Seahunter and Coat of Arms . One card I always suggest for 4 player is Recurring Insight . Like you say merfolk are weak so maybe you should add Ghostly Prison , Collective Restraint and Sphere of Safety . It may seem like a weak card but Crystal Ball is something to consider. Bident of thassa is perfect for you, so you could also add Coastal Piracy . The keyrune and darksteel ignot could be replaced with Mana Vault , Mind Stone , Azorius Signet or Thran Dynamo . If i had to cut things I might cut Inaction Injunction , Portent , Ethereal Armor , Volrath's Curse , Downpour . Good luck.
Before telling you to switch out Galvanic Blast . I would tell you take a look at Mishra, Artificer Prodigy . Which could work in your favor. If you switch out your Plains for swamps. And Switch out Memnite for Glaze Fiend .
About the last card you should really look at is Darksteel Forge . Over all, there a lot directions you can take this deck. To have it be at its strongest with what your doing. You want keep it a dual color. It seems like your aiming for Azorius at moment.