>Why Thassa?

If you always liked the synergistic potential of blue mechanics, but felt reluctant to become the "Nope Guy", then look no further: the incredibly consistent value engine that is Thassa, Deep-Dwelling will scratch all the itches to "put together all the pieces of the puzzle" you might have.
To prove a point, this list is entirely focused on pivoting the other, less known strenghts of the Blue color. Trickery, board interaction and unexpected outcomes are the reuslts that you may expect from this deck.
Which means, no counterspell cards of sort, no infinite turns, no boring cantrips for the sake of cantrip, no cyclonic rift.
Thassa, Deep-Dwelling is an unusual, permanent based general that likes to stick around and play with the very rhythm of the game, often changing tides at the most unexpected moments, turning a threat into a wincon, or discovering a wincon out of nowhere at instant speed.

You'll want to play this deck if:

You like triggers,
You like critical mass synergies,
You enjoy problem solving and puzzles,
You are a Johnny/Timmy player at heart,
You like to play with other people's cards.

I spent alot of time figuring out all the interactions between cards, and still from time to time I manage to find new lines of play that lead to a victory of sorts:
be it suddenly 10+ poison counters to all opponents, or milling and decking two opponents out of three, or stealing every other player's permanents, or finding a way to achieve the classic TOracle alt-win trigger, or attack with an infinite army white 1/1 Bird Tokens (rare, but possible).
from time to time, you'll even win thanks to other people permanents and spells, without the need of any kind of special combo.

Yet again, by choice this deck avoids many of the traditional Blue "staples", in part simply because I don't fancy homogenization of decks through sheer value, over the meaning of the deck itself.
Alas, without further ado, I'll explain how the game plan of the deck looks like.

*Exception would be Glen Elendra Archmage, because combo decks are a thing;

*Still, themtically and mechanically appropriate cards with similar effects are in the list, like Ixidron and Scourge of Fleets for softlock lines against heavy creature boardstates. I am particularly fond of Ixidron because it creates boardstates out of the ordinary.

>Starting Hands

While there is no "must have" early game card combo (We're playing EDH after all), I still have some reccomandations, for the sake of completeness:

KEEPS

Generally speaking, any hand with 3 lands and some early play is good to keep. There are some corner cases tho:

• Hands with 2 lands, at least one "Mana Rock" (Arcane Signet and the like) and some action cards (like e.t.b. creatures) or interaction cards (Sudden Substitution, Commandeer etc):
  In this case you are well set up for a neat curve with a T3 Thassa (potentially) followed up by a value engine of sort, or you are ready to snatch that big spell that Timmy ramped up to.
  Beware, even if we can play the draw-go game thanks to the nature of the color, setting up a bit of board presence is very important in the early game, especially if you need to keep up with the table pace.

• Medium-Fast hands with a t3 Midnight Clock:
  It's better to keep a hand that is capable of doing "something" by t6 even if it's just landing a couple o mana rocks and a small creature or two, if you can land a Midnight Clock early.
  The card is awesome and I highly recommand not to understate the sheer power of refilling the hand in 3 turns. Remember: the clock ticks on eachupkeep, which means that by the 3rd round it stayed on the battlefield, on YOUR upkeep, it will fire off.

• Mediocre hands with Thassa's Oracle, Aether Channeler, Naban, Trophy Mage/Tribute Mage, or High Tide and at least 3 lands:
  This cards are very good and can turn the game unexpectedly. Because of this, you should keep also if you happen to find a "salvageable" hand with Step Through,
  or a one drop and Pyre of Heroes. That last combination is especially powerful and needs to be answered, otehrwise you'll be able to chain creatures and easily setup the board in your favor.

• Carefully consider hands with Throne of Eldraine:
  That card is busted and has incredible set up potential. Paired with ghostly flicker and archaeomancer/shipwreck dowser (or a way to tutor them like pyre of heroes and a 3drop or step through) it gives infinite mana.
  Also, if played on t4 leads a t5 10 mana (which means you can land Omniscience or Myojin of Cryptic Dreams already).

• Unless it's absolutely unplayable, try to keep hands with Leyline of Anticipation:
  The card is a game changer, as it enables playing the game at your end step, maximizing the etb shenanigans from Thassa's triggers.

• While risky, I usually keep starting hands hands with cloning effects, especially Phyrexian Metamorph. The card pull alot of weight during a game thanks to the flickering effects of Thassa, so I find it often being worth keeping.

MULLIGANS

Generally speaking, I like to take advantage of the free mulligan that EDH offers as a format. That's why the deck runs only 34 lands (33+1 MDFC) and even if the possibility of being mana starving is real, I found it to be the "sweet spot" in terms of balance between consistency and card density.
In my testings the deck doesn't really needs more than 4 to 5 lands in play to run smoothly, thanks to all the mana rocks and ways to enhance and multiply the mana available.
That said:

• Mulligan any hand with 4+ lands and no way to impact the board early game. What you want to avoid at all costs is for Thassa to be your first creature on the board. Save very few exceptions (the table's expected game pace is very slow, or you have some very strong lead with Leyline of Anticipation as a t0 play), you want to have Thassa's end turn trigger to start generating value as soon as she comes down.

• Mulligan also if 2 lands and no mana rocks. You CAN risk if you have a 1 drop and a 2 or 3 drop, but I would recommand it only if you see some very powerful line in hand already. Otherwise, being stuck for the rest of the game with 2 lands and a dork is not worth it. Don't get tricked by Frantic Search, that can tempt you to keep risky hands with the promise of digging cards for free.

>Early Game

The first 1-3 turns plays very similarly every game:
Either you will be putting down artifacts, or small e.t.b. creatures, setting up for the Thassa play. It's in your interest to be wary of playing "scary" creatures early, like Thassa's Oracle or Naban, dean of Iteration. Thassa's Oracle is a powerhouse card that can be used in a pinch to dig for fuel if needed.
Basically, what you want to do early in the game is to see as many card as possible, to put yourself ahead in the resource game.
I would not advise to play Pyre of Heroes on curve, if you have another choice that's not Prologue to Phyresis (PtP for short). PtP is not a card to be played early, it's a win condition. If you feel you have to play PtP early, is a sign that you kept a bad or slow hand.
If the game flows slow and you see there is no rush to develop, you can play Thassa on an empty board. Otherwise, prioritize board presence. Often you'll be the target of free early attack steps, because unless strictly necessary, you don't want to kill your creatures as almost each of them has the potential of escalating into a combo win or into becoming a strong value engine.

>Winning the Game

This is the fun and difficult part of the deck. How does the deck wins?
Generally speaking, you have to meet one or more criteria to win the game:

• You have a way to recur spells from the graveyard;
• You have a way to make infinite mana.

After meeting such requirements, you can choose to end the game with one of the following:

• Milling out your opponents' decks (either by flicking 100 times Overwhelmend Apprentice, or by casting Drown in Dreams X=100 twice.);
• Drawing your deck and casting/flicking Thassa's Oracle;
• Stealing all the permanents on the battlefield;
• Giving the table 10 poison counters (that can be done either by casting PtP once and then playing 9 or less non-creature spells with one or more Flux Channeler in play, or by Proliferating with Gitaxian Anatomist ETB 9 times, or by recurring PtP 9 times);
• Having the possibility, flicking instant speed Aether Channeler for infinite 1/1 flying Birds - this is the riskiest line of play because the table can answer through combat tricks, but it's still an option.

One of the cleanest line of play that can happen would be:

Shipwreck Dowser, Arcane Signet, Throne of Eldraine, Ghostly Flicker, Aether Channeler, 2 Island. Topdeck: Island

T1 Island
T2 Arcane Signet
T3 Aether Channeler, drawing a card. If needed you can create a Bird to chumpblock and protect yourself, or bounce dangerous permanents to buy time
T4 Throne of Eldraine into Thassa, Deep Dwelling. Again, here you can use Channeler ETB to it's best use. I had the luck of finding Spellseeker, which enabled a t5 win.
T5 Tapping Throne for {U}{U}{U}{U} , Spellseeker to find High Tide, casting it for the remaining {U}. Now we have 11 effective Blue Mana (10 from lands, 1 for signet).

• We can use 5 to cast Dowser, in response to it's ETB we can cast Ghostly Flicker for {U}{U}{U}, targeting Throne and Dowser.
• The second ETB trigger from dowser resolves, returning Ghostly Flicker to our hand. With still the first ETB trigger on stack, we can tap throne for {U}{U}{U}{U} and use {U}{U}{U} for Flicker, leaving us with {U}.
• we can repeat the loop to net any amount of {U}, and then switching targets from Throne and Dowser to Dowser and Channeler, bouncing all non-land permanents, creating any number of Bird Tokens and drawing our deck.

Had been Leyline of Anticipation in play, we could pull this off at instant speed.

>Card Analysis (w.i.p.)

>CREATURES