Introduction
You know what? Exploring is powerful. Like, really powerful, actually. Explore has been an important part of its standard environments, not really as a way to win the game, but as a reliable way to generate additional value and getting a little bit ahead at a time. Yet commander has lacked the tools to do this, until Lost Caverns of Ixalan's Commander set - until Hakbal of the Surging Soul.
Now, Hakbal allows us to do exactly what explore has been doing in standard and limited formats but scales it up to commander power levels. The only thing you really need to make Hakbal into a viable deck for upper mid tiers of power level is to include ways to get Hakbal out early, refill your hand and keep your commander from being removed too many times - then you just fill the rest of your deck with decent merfolk and you're good!
So let's take a look at this specific deck; how it accomplishes these things, and how I have prioritized to take the deck in a slightly more powerful direction.
The Basics
Mana
Like we've gone through really quickly, there are some key things to do to make Hakbal of the Surging Soul hum. The first of these is to get our commander onto the field as soon as possible. If we want to avoid the most expensive mana generators that would allow us to ramp using free spells, this means we want to play at least one ramp spell on either turn one or turn two.
To accomplish this, I have focused on ramp that is either strapped on to a merfolk, provides us with mana advantage that is harder to remove with sweepers, or gives us some additional benefit apart from generating mana itself. Kiora's Follower, Sea Scryer and Stonybrook Banneret are merfolk, Arcane Signet, Farseek, Nature's Lore, Rampant Growth, Talisman of Curiosity and Three Visits provide sweeper-proof ramp, and Incubation Druid synergizes with +1/+1 counters and can allow us to generate infinite mana.
Blighted Woodland, Merrow Reejerey, Nicanzil, Current Conductor and, most importantly, Hakbal of the Surging Soul himself also provide ramp later in the game and will help us keep pulling ahead. They are not helpful in getting us off the ground (seafloor?), however, so I have made sure to include ten ways to ramp us into a turn three Hakbal.
Notable Mana Excludes
This deck does not run any one mana ramp spells. While Sol Ring might have a place in the deck, any one mana dork would allow us to play a dork on turn one, a three drop on turn two and still get Hakbal out on turn three. I have reasoned that reliable or synergistic turn two ramp out weighs the raw power of dorks, but you might well get even better results including the dorks - and if you are running expensive fast mana you should certainly aim to play Hakbal on turn one or two whenever possible.
Cards
Secondly, we want to have good ways to refill our hand with cards over the course of the game. The card advantage spells in the deck are assumed to be played after getting Hakbal onto the battlefield, and are therefore leaning into high impact over low cost.
The standout card draw spell in the deck is Tatyova, Benthic Druid, since we can play her as soon as the turn after Hakbal to generate an additional explore and since she turns our commander's attack trigger into "draw one or play a land, gain a life and draw one". We will be getting two land drops most turns, so drawing two additional lands each turn is clearly a very good thing. Apart from Tatyova, we are running spells that care about us playing lots of merfolk (Distant Melody, Emperor Mihail II, For the Ancestors, Kumena, Tyrant of Orazca, Realmwalker and Seafloor Oracle) and spells that care about the +1/+1 counters generated through exploration (Bred for the Hunt, Inspiring Call and Sage of Fables).
Notable Draw Excludes
We are not running any taxing draw cards (such as Mystic/Rhystic), any cantrips or any "draw X"-spells in this deck. The taxing draw spells would work very well with our highly interactive game plan, and cantrips would benefit from us often wanting to hold up interaction. Yet, these spells do either too little or too much on their own to be clearly worth the include. Taxing our opponents will likely make us a target since we will also be amassing a quite threatening board very quickly, and the cantrips simply seem hard to justify spending card slots on.
The more egregious exclude here seems to be spells that make any player draw X cards, since we have several ways of generating infinite mana in the deck and these spells could serve as an alternate win condition. It seems we are already quite able to turn massive mana advantage into a win, however, and these spells don't quite offer the same level of continual value that most of our drawing effects do. You might well do better making a few swaps regarding this, though.
Interaction
Thirdly, we need to keep our game plan safe. We do this by a considerable interactive package on a budget, running 10 counterspell effects (e.g. Counterspell, Mana Leak and Vodalian Hexcatcher), 9 targetted removal spells (e.g. Decisive Denial, Imprisoned in the Moon and Thieving Skydiver) and 4 sweepers (Evacuation, Fade Away, Filter Out and Wave Goodbye). We also generate absurd amounts of land drops if we can get Hakbal attacking safely a few times, meaning we are quite capable of rebuilding if we need to.
The Merfolk
With the basics covered, let's take a look at the includes and excludes among our merfolk in the deck.
Some merfolk in the deck are not very surprising. We run Nicanzil, Current Conductor and Topography Tracker that improve our exploration, we run Kumena, Tyrant of Orazca and Lullmage Mentor who create value from having bunches of merfolk on the board, and we run Deeproot Pilgrimage and Deeproot Waters that give us more merfolk to explore and do combat with.
We are, however, not running most of the traditional merfolk lords, such as Lord of Atlantis & co. These creatures simply don't do enough for us in a slower format with more opponents and higher life totals. Hakbal already grows our team while also letting us dig though our deck and secure our land drops - and if we can't take much advantage of the minor bump to power that would be extremely powerful in other formats, it simply seems more advantageous to fill our deck with more impactful spells. (Even the three included lords are very much first in line for any cuts, where Vodalian Hexcatcher seems to give us the most by being able to counter spells and having flash, Merfolk Sovereign being kinda nice by enabling us to ensure Hakbal can get an attack in, and Merrow Reejerey hanging on for dear life due to at least being able to generate some additional mana or maybe help a combo go a little smoother.)
We are also running a bunch of seemingly random one mana merfolk - we are not even running the one mana merfolk that explore or avoid blockers! Instead, we are focusing our one mana creatures on merfolk that are able to tap (themselves or another merfolk) for an activated ability. Mothdust Changeling, Reef Shaman, Tidal Visionary, Tidal Warrior and Tideshaper Mystic all do just this, and we don't actually care much about what effect we get from tapping a merfolk - we just want them to tap.
The Combos
But why do we want our merfolk to be tapping themselves (except for the obvious beauty in merfolk self love)? We want them to be combo pieces along with Deeproot Pilgrimage and Intruder Alarm and generate infinite merfolk tokens for us, this being one of a small group of combos the deck runs to increase its power level towards a lower high power deck. Since it is difficult to tutor for enchantments in simic colors, we will struggle to be efficient in pulling off combo wins. But when we are exploring many times each turn we dig through our deck surprisingly quickly, and we will find ourselves locating the combo pieces more often than we think simply by making sure we have plenty of merfolk on the battlefield while we stop our opponents from stealing the win.
To end this deck description, I will list the main combos in the deck and describe them briefly. Feel free to reference this list if you'd like to playtest the deck, feel free to suggest other synergistic combos to add or ways to reliably locate our enchantments, and I hope you like my take on Hakbal of the Surging Soul!
Listed Combos
Two Mentions
This deck runs two more very important cards in Simic Ascendancy and Wildgrowth Walker. It is only fair to quickly mention that these cards can single handedly secure us wins. Simic Ascendancy can often generate 20 counters before our next upkeep if we have Karn's Bastion on the battlefield, and Wildgrowth Walker will often gain us around 20 life from going to combat a single time with Hakbal on the field, putting us well beyond the reach of many more aggressive decks instantly.
Cheers, and have fun playing our magical little game of cards!