VALUE! VALUE! VALUE!

They oughtta start calling this game "Versatility: The Gathering," what with all the variety the cards here bring to the table. Siren Stormtamer, Merfolk Trickster, Thassa, God of the Sea, and Master of Waves can, with a little luck, come out perfectly on curve, and with a Cavalier of Gales on turn 5 or 6, a win is more or less in the bag. This is something like a midrange deck wearing a control hat, with disruption like your variety of counterspells, Blustersquall, and Frogify functioning a lot like direct removal does in more traditional midrange lists. 

YOUR STUFF:

Thassa, God of the Sea is just a tremendous card in blue, and it's by far the most expensive one on this list (mainly because it's never been reprinted). The scry every turn is amazing, even when Thassa isn't a creature, and when she is she can make herself unblockable and chunk a quarter of your opponent's life total on turn 4, which is amazing in a deck that can still exert a lot of control. Mu Yanling, Sky Dancer is another great source of passive value, neutralizing threats and pumping out 4/4 fliers as you choose. She comes in at only 2 loyalty, but if you can get her up to her ultimate you more or less win the game, and her +2 and -3 are much more important in a midrange deck like this one. The tokens Yanling creates are Birds, which is great because I love Birds, but they're also Elementals, meaning that they're 5/5s with a Master of Waves out. The Master's status as an Elemental lord (despite being a Merfolk itself) is nice, but it's not why there's a full playset of it in any functional Devotion to Blue deck. It's because of the frequently daunting army of Elementals that come with it, and with cards like Siren Stormtamer, Witching Well, and Merfolk Trickster populating the low-drop slots, your devotion should already be pretty high before Master of Waves comes out. The protection from red is also a great plus.

Cavalier of Gales is the curve-topper and biggest win-con in the deck, drawing you a bunch of cards, swinging for huge damage, never truly dying unless exiled, and even appreciating a buff from Master of Waves. Once you have one of these out, you're in pretty good shape for the mid-to-late game. Merfolk Trickster is a really efficient 2/2 with flash and a major upside for 2, and it's even better in Devotion because of its double-blue casting cost. Siren Stormtamer rounds out the creatures with a counterspell strapped on. A 1/1 in the air for 1 is already not terrible in blue midrange, and sometimes the threat of a counterspell your opponent knows is there is more effective than an ordinary counterspell in your hand. As Foretold functions a lot like Aether Vial does in Modern Merfolk, but with a higher mana cost and even more versatility, which makes it fantastic in Pioneer. Bident of Thassa is super affordable and a great late-game drop to refill your hand with ease, especially with Master of Waves and your fliers. It's allso a cheap source of disruption that synergizes nicely with the deck's other tap effects. Witching Well scries you cards, gives you midgame devotion, and can draw you cards later on if you need them, and Chart a Course just pads your hand out without reservations. Skyline Cascade is a niche land choice in mono-blue that can keep your opponent's things tapped, too, which is an interesting option.

MESSING WITH YOUR OPPONENT'S STUFF:

Speaking of tap effects, Blusterquall is a great, cheap instant from RTR that takes advantage of Overload to scale up into the late game in a way not many cards really do. Frogify is a crippling debuff that's hard to remove in a meta that runs very little enchantment removal, and it boosts your devotion to boot. What you're really rocking in the search for answers here, though, is your robust suite of counterspells: two each of Spell Pierce (to disrupt your opponent's early game plays and ruin combos), Thassa's Rebuff (which gets stronger as you gain Devotion and is usually a superior Mana Leak), and Wizard's Retort (which is usually just Counterspell with all the Wizards we have here). 

YOUR SIDEBOARD:

Most of the time, midrange decks employ sideboards that allow them to pivot to more dedicated aggro or control strategies, but in mono-blue, aggro is honestly not that viable of an option. What we have instead are enough counterspells, support for those counterspells, and aggro-color hate to dedicate the deck to a creature-heavy control strategy that banks more on its big win-cons than on playing on curve. Pithing Needle lets you proactively go after specific spells that are harshing on your game plan, and is probably just the universal standard for a good sideboard card in Pioneer.

TL; DR:

Mono-blue offers some really worthwhile options in midrange that more traditional colors (Jund, Simic, Abzan, etc) don't. Substituting creature control for creature removal, using versatile, efficient creatures of its own, and taking advantage of mechanics that benefit mono-blue disproportionately (Devotion) and cards that only mono-blue can really run (counterspells, creatures with heavily blue costs, Thassa), this list is fully aware that blue is the best color in the game and wants to take advantage of it. Using a counterspell-rich sideboard, it can pivot comfortably into a full control deck and beat other midrange strategies as well. It also comes with the benefit of never having to worry about mana fixing or being color-screwed (like all mono-colored decks), and it beats aggro (especially red aggro) much more decisively than most midrange strategies do. Watch out for other control decks, though - Mystical Dispute is a threatening sideboard option against anything heavy in blue, and this deck's best option against hardcore control is to turn the game into a mirror match or hope its opponent draws poorly, which isn't particularly consistent. Maybe that Sphinx that stops people from countering your spells would fit in here?