Biomancer's Familiar & Training Grounds
I personally consider these two cards must-haves in this kind of deck. They help out so much. They are a secondary win condition that let us put all the mountain into play one by one.
If you have both in play Thrasios, Triton Hero ability cost only mana and that will get out of hand realy quickly.
The Tabernacle at Pendrell Vale
A perfect interactive land. We need to have a huge land count for this deck to work. We also don't have many creatures in this deck. We are also ramping an putting a lot of lands into play. We are not affected that much from this land.
Narset's Reversal
Is an amazing card! But it cost mana. This is a 2 cost card that will make us loss 2 life during the Ad Nauseamcombo run. Double blue is also tricky in a deck like this. is possible but not always guaranteed. We can play Pact of Negation because we actually don't need to have for if next turn never comes.
Mox Tantalite
Suspend-3 hmmmm. I am not so sure about this card in general. But it has mana cost. Or well it dosen't have a mana cost. And when you play a commander like Thrasios, Triton Hero you don't need to worry about having to much mana. It will also help with Mox Opal. If this card is in your opening hand you have a Thrasios, Triton Hero activation on turn 3. But if you draw it later. Then it isen't that great.
Life from the Loam
This deck is bad at utalizing this card in general. It not built arround this card in general. Sure we have a lot of lands in this deck. But in general I don't always include this card in my list. Its not a must have. You could cut it. But It is good when Thrasios, Triton Hero don't get to do anything. Or if there is a Cursed Totemin play. This is a back up merfolk commander alternativ you could say.
Mox Opal
would be awesome if we could include it. But we don't have that many artifacts to make sure we get metalcraft. But we could include a bunch of artifacts to make this happen. this mana cost artifact will open up deck a lot more.
Ancestral Vision
During our combo run with Ad Nauseamwe woll lose 0 life from this in our deck! But we can cast it for to draw 3 cards!!!! ... after 4 turns ... In the end that is okay. Its not great and few decks play this card. This format is a fast format. You might cast it and the game could be over befor it resolvs. Still, its not a bad card. Every cEDH deck could actully play it. With color identity of blue that is. So in the end this is a card you could include if you want to. Its also good to have cards that will help us out if we don't get going with Thrasios, Triton Hero.
Scavenger Grounds & Bojuka Bog
Whitout lands with interaction this deck would be hard to assemble. We need a a high land count to give Ad Nauseamthe chance of drawing the entier deck. The more lands you have inside this deck the lower your life can go and you still have this option. These two lands have good and working interaction. They also work wonders with Crop Rotation.
Deathrite Shaman
This is everything this deck wants in one card. It is a mana dork and this deck wants mana. It has graveyard interaction against our opponents. We can exile a creature card from an opponents deck they wanna bring back into play with something like Animate Dead. But we can also casually exile an instant or sorcery to force each opponent to lose 2 life.
This card is a group slug, mana dork, interactive card. Supprisingly the gain 2 life is helpfull.
Drop of Honey
Valakut, the Molten Pinnacle is excellent at killing medium large creatures. But killing large amounts of tiny creatures (1/1s) is difficult. Also it takes some time before this vulcano gets active. This green enchantment helps out at the early stage of the game vs decks that creature swarms. Slowing down the game. We are not playing a creature heavy deck and we can survive without our commanders. A perfect 1cost card for this deck.
Field of the Dead
You know what. Why not? Its not great but we could explode with this land. It could be a secondary semi wincon. A beatdown wincon. We can't interact with opponents creatures like we do with Valakut, the Molten Pinnacle. But we can send our zombie army into our opponents face and win. It taped for colorless and that is a BIG problem. Color fixing is already tricky so it is a risk including this land.
Pithing Needle
is a great cost card that makes Mox Opalmore playable. It will help us achieve metalcraft. Its also great during the Ad Nauseam combo run. We are looking for low cost cards with interactions. But also Caustic Caterpillaris quite a common card you could run into during competitive EDH games. This bug can kill Manabond before we get to end step. This little needle stops this for only mana!
Natural State
Most deck only play Nature's Claimwhile we play both. The answer is one simple card name: Blood Moonwitch is the best counter towards this deck. And we respect that card so much that we play an extra copy of Nature's Claim. Weaker, but will still do the job.
But this deck is really good at slaughtering creatures with this mountain volcano Valakut, the Molten Pinnacle. And there is a good amount of lands with graveyard interaction. Left is artifacts and enchantments. So it kinda makes seans to to include Natural State just for better interaction balance.