A Legendary Ocean decks rely on their Field Spell Card, "A Legendary Ocean", hence the name. "A Legendary Ocean" decreases the Level of all WATER monster in both players' hand and field by 1. This allows summoning of powerful WATER monsters like "Giga Gagagigo" without any tributes, and decreasing the number of tributes needed for even stronger monsters, like "Levia-Dragon - Daedalus". Having their Levels decreased, WATER monsters are also immune to cards like "Gravity Bind", so it will only lock down their opponent, and also supported by "Hammer Shark" and "Hydro Pressure Cannon". "A Legendary Ocean" is also treated as "Umi", wich is required for the effects of many WATER monsters.
Daedalus Method: One way to use A Legendary Ocean is with the Levia-Dragon - Daedalus or its superior, the Ocean Dragon Lord - Neo-Daedalus Both monsters' effects require A Legendary Ocean or Umi to be activated. As a result, the entire field is destroyed except the Daedalus, leaving the user a clear field to perform direct attacks on the opponent. Normally, people would prefer A Legendary Ocean for this operation in contrast to Umi. This is because summoning the Levia-Dragon - Daedalus would only require one Tribute. Others prefer to use the card Big Wave Small Wave to summon the Daedalus, which is a smart addition to Water Decks regarding these tactics. This method is preferably combo with the Stall Method to protect yourself when you aren't able to draw Daedalus or unable to summon it yet.With this approach, the best thing to do is swarm the field and then activate Aegis of the Ocean Dragon Lord to protect the monsters on your field, use Daedalus's ability and destroy the field (except for your monsters), and you should have a field of monsters left to go for a direct attack and an instant win.
Water monsters can also be played to destroy monsters, spells, and/or traps to perform multiple direct attacks. In this case, A Legendary Ocean does not serve as big of a role whereas cards like Umiiruka may help a little more since it adds more significant ATK strength.
Abyss Soldier and Penguin Soldier Method: This method uses Abyss Soldier and Penguin Soldier to send cards back to the opponents hand and then allowing you to attack directly. Keep in mind that Gale Lizard also sends any monster to your opponent's hand. Combined with A legendary Ocean Gale Lizard has 1600 attack and can easily be powered up. Abyss Soldier is a powerful monster on his own with 2000 attack while A Legendary Ocean is on the field. Combine this method with swarming tactics like Aqua Spirit's effect or Mermaid Knight's double attack effect to hit for massive damage in one turn while attempting to shut down any chance of your opponent getting out powerful monsters.
Hydrogeddon: If your opponent is simply summoning defense position monsters to try to stall until they get better cards then you can pretty much wipe out their entire side with Hydrogeddon. 1800 attack (assuming you have A Legendary Ocean on the field) beats the defense of most of the common monsters out there, and when you do destroy a monster you can special summon another Hydrogeddon.
Deep Sea Diva Swarm: If you want to increase the attack of your monsters easily, or just want to get a low level monster with a good amount of attack points, use Deep Sea Diva to special summon a level 3 or lower sea serpent type monster out on the field. The two key cards are Mermaid Archer which you can equip Deep Sea Diva to to raise it's attack, or you can special summon Spined Gillman which raises the attack of all: Sea serpent, aqua, and fish types you control by 400 points. It is also a good idea to have Star Boy which also raises the attack of all water monsters on the field and lower any fire monsters. This strategy works well if you have Double Summon so you can lay out more Deep sea Divas and swarm you opponent with low leveled, high attack point monsters. This can work well with Gravity Bind or Level Limit - Area B to keep your opponents monsters grounded while you increase your attack and avoid the stall effects. If you run low level synchro cards, Deep Sea Diva is a tuner that that can easily help you get what you need.
Frog Decks are based around the Frog archetype. They consist of low-level Aqua-Type WATER monsters. Although weak on their own, they are mostly used for Tribute fodder and Xyz Summons. The main monster, "Treeborn Frog", is Special Summoned during each of your Standby Phases as long as there are no Spell/Traps or other "Treeborn Frogs" on your side of the field. "Swap Frog" can Special Summon himself from your hand by discarding 1 WATER monster, and can return himself to the hand as well. When it's summoned, it can dump another Frog to the Graveyard, like "Treeborn Frog" or "Ronintoadin". "Ronintoadin" can banish 1 Frog monster from your Graveyard to Special Summon himself from the Graveyard. He is mostly used for Rank 2 Xyz Summons, like "Gachi Gachi Gantetsu" and "Number 96: Dark Mist".
Frognarch decks use the Frog Engine to summon Monarchs, triggering their effects. The first step is to send "Treeborn Frog" to the Graveyard, usually with "Swap Frog"'s effect. He is then used as tribute in the following turns for the powerful Monarchs. "Caius the Shadow Monarch" banishes 1 monster on the field when he's Tribute Summoned, and "Mobius the Frost Monarch" can destroy up to 2 Spell/Traps on the field. Another ace monster, "Light and Darkness Dragon" can completely lock-down the game, negating any effect that activates, including "Treeborn Frog". "Enemy Controller", for being a Quick-Play Spell Card, can be used when "Treeborn Frog" is summoned, tributing him to take control of an enemy monster, and since it's still the Standby Phase, "Treeborn Frog" is Special Summoned again, thus giving you control of an enemy monster until the End Phase for no cost.
Traps aren't recommended as they block "Treeborn Frog"
Lancer Frogs decks function like Frognarchs, however, they focus on their ace monster, "Sea Lancer". "Sea Lancer" can equip any number of banished WATER monsters to himself, giving him a 1000 ATK boost, and negating its destruction by sending one of the equiped monsters instead. "Ronintoadin" is essential to this deck, as he is the one who banishes Frogs for "Sea Lancer". Cards like "Aqua Jet" and "Horn of the Phantom Beast" are used to support "Sea Lancer".
Traps aren't recommended as they block "Treeborn Frog"
Coelacanth/Fish Synchro Deck
Coelacanth Decks focus on Synchro Summoning through the effect of their main monster, "Superancient Deepsea King Coelacanth". "Coelacanth" can discard 1 card to Special Summon as many Level 4 or lower Fish-Type monsters from the deck as possible. The main monsters to be summoned are "Royal Swamp Eel", for being a Level 4 Tuner and "Oyster Meister", for Special Summoning a "Oyster Token" to be used for a future Synchro Summon. Usually 2 "Royal Swamp Eels" are summoned, then "Oyster Meister" and a Level 4 Fish monster. Then the player can go for Synchro Summons from Level 7 to Level 9, wichever he finds best.
Gishki Decks use the Ritual Summon based Gishki archetype to quickly summon powerful Ritual Monsters. Their main Spell Card, "Gishki Aquamirror", can only Ritual Summon by tributing monsters with a level equal to the Ritual Monster, but it can recycle itself by shuffling itself to the Deck (while it's in the Graveyard) and adding a Ritual Monster from the Graveyard to the hand as well. The two main non-Ritual monsters of this archetype are "Gishki Vision" and "Gishki Shadow", that can discard themselves to search a Gishki Ritual Monster or a Gishki Ritual Spell, respectively, can be used as the whole tribute for the Ritual Summon of a WATER monster, and can be recycled with "Salvage". All of the Gishki monsters have even levels.
The Zeal Gigas/Gustav Max OTK uses the power of "Gishki Zielgigas" and "Superdreadnought Rail Cannon Gustav Max" to finish the opponent in one turn. First, to clear the field, one can use "Zeal Gigas'" effect to shuffle a card in the Deck by paying 1000 Life Points and drawing a Gishki monster. "Zeal Gigas" can be Ritual Summoned many times by recycling it with "Gishki Aquamirror" and using the "Zeal Gigas" already on the field as the tribute. After the field is clear, and you have at least 2 "Zeal Gigas" on the field, attack directly, then Overlay for "Superdreadnought Rail Cannon Gustav Max" to finish the enemy with its effect. This deck is very versatile, and it works well even if you can't OTK the enemy.