Tidal, Dragon Ruler of Waterfalls (Tidal) is the Water deck's boss monster. It has 3 effects, but you can only use one per turn. Tidal's stats make it one of the strongest boss monsters in the format, and as long as you can avoid banish-effects (Bottomless Trap Hole, Dimension Prison), it floats whenever it's on the board. This makes it great for clearing your opponents field and regaining lost momentum, as well as for finishing the game.
Its first effect allows you to banish 2 WATER monsters from your Hand or Graveyard to Special Summon it from your hand or Graveyard. It will return to your hand at the end of your opponent's next turn, allowing you to reuse it every turn. This effect unifies Water decks and means the more WATER monster you include, the easier it is to repeatedly Special Summon Tidal.
Its second effect allows you to search another WATER Dragon-type monster from your Deck the turn it is banished. In practice, cards like Bottomless Trap Hole and Dimension Prison will not trigger this effect, because they would usually be played the same turn you Special Summon Tidal (remember the once-per-turn condition). We also designed the format to have limited options for Tidal to search, as a way to keep its power in check. As of now, Tidal can only search another copy of itself.
Its third effect allows you to discard itself and another WATER monster to send a monster from your Deck to the Graveyard. This seems like a costly effect, but it's actually not. First, Water decks have tons of WATER monsters that trigger their effects on discard. Second, Tidal can Special Summon itself from either the Hand or the Graveyard. Third, WATER decks tend to have many important monsters they want to send to the Graveyard, such as Sinister Serpent or Treeborn Frog. Keep in mind, however, you cannot Special Summon Tidal the same turn you dump it for this effect, because of the hard "once per turn" clause.
Mermail Abysspike (Pike) is one of the Water deck's main starter monsters. His effect is to search for a Level 3 WATER monster (targets: Elemental HERO Ice Edge, Genex Undine, Graydle Cobra, Mermail Abyssgunde, Mermail Abysslinde, Snowman Eater). His stats are midrange for Level 4 monsters in the format.
Ideally, you want to discard something like a Sinister Serpent, Mermail Abyssgunde, or Atlantean Heavy Infantry to get a +1, but this is not necessary on your first turn. Pike can get you set up to make a strong move on your second turn, and he quickly fills the Graveyard for Tidal and Abyssgunde.
Pike is a great monster to use to make a Rank 4 XYZ monster, because he pays for himself when summoned.
Note that Pike gets his effect on either a Normal Summon or a Special Summon. This means whenever he is Special Summoned you have the option to trigger his effect (triggers: Call of the Haunted, D.D.R. - Different Dimension Reincarnation, Premature Burial, Mermail Abyssgunde, Mermail Abysslinde).
Mermail Abyssturge (Turge) is the Water deck's main recycler. His effect is to return a Level 3 WATER monster from the Graveyard to the hand. This makes Turge the opposite side of the coin with Pike. Most of the tips for using Pike also apply to Turge.
Ideally, the Water deck would like to summon Turge and Pike in sequence, since they compliment each other. They are commonly used together to make Rank 4 XYZ as well.
The most important monster that Turge can recycle is probably Mermail Abyssgunde, as this allows the deck to continue to make free discards and Special Summon an extra monster. Mermail Abyssgunde being semi-limited makes this even more important.
Mermail Abysslinde (Linde) is the Water deck's main recruiter. Her effect is fairly robust, because she will trigger whenever she is destroyed and sent to the Graveyard, not just in during Battle. This is important in Pioneer Format because of cards like Diamond Direwolf, Ryko, the Lightsworn Hunter, The Phantom Knights of Break Sword, and Torrential Tribute.
Linde will summon any "Mermail" monster from the deck except another copy of herself (which is an unfortunate drawback for a recruiter). The most notable targets are Pike and Turge, because their will trigger when they are Special Summoned. In an emergency, you can recruit Gunde and use her as tribute fodder or XYZ material.
Linde works well with the deck, and helps to fuel Tidal.
We all know how Sinister Serpent works from Goat Format, but it's safe to say it's a key piece for the Pioneer Format Water deck to function. Not only can cards like Tidal or Undine dump it into the Graveyard it for free, but it can be used to activate many powerful WATER monster effects (Abyss Soldier, Abyss Warrior, Mermail Abysspike, Mermail Abyssturge, Tidal, Dragon Ruler of Waterfalls).
Sinister Serpent is limited because of how strong and focal it is to the many deck's game plans, not just the Water deck.
Atlantean Heavy Infantry (Heavy Infantry) is a key discard option for the Water deck. Often, the Water deck wants an early WATER monster to discard and Sinister Serpent might not be available. The Water deck is frequently discarding multiple cards in a single turn as well, meaning Sinister Serpent isn't enough. Heavy Infantry fits the bill here.
Heavy Infantry's first effect, which allows it to special summon a Level 4 Sea Serpent from your hand, is rarely relevant in Pioneer Format. The only legal target for this effect is Armed Sea Hunter.
Heavy Infantry's second effect is much more important, and it will trigger whenever it is used as a cost. This means it works with all of the Water monsters that discard it as a cost for their effects (Mermail Abysspike, Mermail Abyssturge, Abyss Soldier, Abyss Warrior, Tidal, Dragon Ruler of Waterfalls), as well as Genex Undine when it sends Heavy Infantry from the Deck to the Graveyard to search for Genex Controller.
In practice, this makes Genex Undine much more versatile, giving it the option to either search a card from the Deck or blow up a card on the field.
Genex Undine (Undine) is one of the Water deck's main starter monsters. Its effect allows you to send any Water monster from your Deck to your Graveyard as a cost to search for a Genex Controller. However, the cost is actually the main benefit of playing Undine, since it can allow you to send cards that activate in the Graveyard like Atlantean Heavy Infantry, Sinister Serpent, Treeborn Frog, and Tidal, Dragon Ruler of Waterfalls.
Because we are playing by Goat Era Rules, you are allowed to "fail to find" for Undine's effect. This is key, because you can use Undine to dump WATER monsters without needing to play the bricky Genex Controller, or if you are out of Genex Controllers.
In practice, Undine often plays like the Water deck's version of Armageddon Knight. These types of effects are very important in Pioneer Format. They allow for fun toolbox-ing and consistency.
Note also that Undine is a legal target for both Pike and Turge.
Graydle Cobra (Cobra) is a nice option for Water decks. Its effect basically makes it a better version of Snatch Steal when it is destroyed and sent to the Graveyard. Note that unlike Snatch Steal, Cobra will destroy the equipped monster when it leaves the field, instead of giving it back. Note also that it is effectively immune to many trap cards, however, not Dimension Prison.
One key line in Cobra's effect is that you can choose which monster to steal. It does not have to steal the monster that destroyed it in battle if your opponent has another legal target. This means you can attack a weaker monster to steal a big monster, and your opponent will lose their best monster is they try to attack it on their turn. Because Cobra's effect occurs in the Battle Phase, if it's your turn, you can attack with the monster you just stole.
Cobra is a solid target for Pike and Turge, but not an engine piece. You can probably get away with playing 1 copy as a toolbox-able answer.
Abyss Soldier has an incredibly strong effect, and it obviously has a lot of synergy with the Water deck. As a reminder, you can discard one WATER monster to return one card on the field to its owner's hand. Abyss Soldier also has pretty strong stats for a Level 4 monster.
Notably, in Pioneer Format, Extra Deck monsters are even more prominent than they are in Goat Format, so you can pretty much get good value out of Abyss Soldier's effect almost every turn. For this reason, we have Abyss Soldier semi-limited.
This format also has a lot of fun Abyss Soldier combos. Some notable reuse combos include Gameciel the Sea Turtle Kaiju, Graydle Cobra, Fiendish Chain, Genex Undine, Premature Burial, and Call of the Haunted.
Abyss Warrior is a nice side deck option for Water decks. At the cost of a WATER monster discard, Abyss Warrior can put 1 monster in either player's Graveyard on either the top of the bottom of the Deck. Abyss Warrior also has pretty strong stats for a Level 4 monster.
There are lots of decks in Pioneer Format that rely on having specific monsters in the Graveyard, and Abyss Warrior does a great job at disrupting those decks.
Notably, this effect can be used offensively to put a bad draw on top of your opponent's deck, much like Phoenix Wing Wind Blast. In some situations, you may want to stack the top of your Deck with a specific monster from your own Graveyard.
Gameciel, the Sea Turtle Kaiju (Kaiju) is an interesting option for the Water deck. It has some nice, aggressive combos with Abyss Soldier and Graydle Cobra. Its effect essentially allows you to tribute summon it to your opponent's side of the field using one of their monsters as a tribute, effectively making it a modern version of Lava Golem. However, unlike Lava Golem, Kaiju is live more frequently and allows you to summon other monsters the same turn.
In practice, this card is better in the Advanced Format of Yu-Gi-Oh! than it is in Pioneer Format, because untargetable and undestroyable monsters are less common and less threatening here. It could still be nice as either a side deck option or an option for a creative strategy.
Armed Sea Hunter is a nice aggressive option for the Water deck. Its first effect allows it to negate the effect of any effect monster it battles, however, it requires you to have another WATER monster with it on the field. Its second effect allows it to survive if you tribute another WATER monster.
The best way to get a good use out of Armed Sea Hunter is to utilize the Water deck's ability to Special Summon. Some nice combos to cheat out a partner for Armed Sea Hunter include Atlantean Heavy Infantry (first effect), Silent Angler, Tidal, Dragon Ruler of Storms, and Treeborn Frog.