A Legendary Ocean Deck
Numerous water decks rely on the Field Spell Card "A Legendary Ocean" to perform powerful effects as discussed below.
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 "Umi" to be on the field (which "A Legendary Ocean" is treated as). As a result, the entire field is destroyed except the "Daedalus", leaving the user a clear field to perform direct attacks on the opponent. If "Abyssal Kingshark" is on the field when you activate the effect of "Daedalus", the "Kingshark" is not destroyed. Normally, people would prefer "A Legendary Ocean" for this operation in contrast to "Umi", because Summoning "Levia-Dragon - Daedalus" would only require one Tribute. Others prefer to use the card "Big Wave Small Wave" to Summon "Daedalus", which is a smart addition to WATER Decks using these tactics. Another good idea is using "Warrior of Atlantis" to search out "A Legendary Ocean".
Decrease Level Method 1
Another way to use "A Legendary Ocean" is to utilize its Level decreasing effect on WATER monsters on the field and in the hand. Strong Level 5 WATER monsters such as "Giga Gagagigo", "Terrorking Salmon", or "Catapult Turtle" can then be Normal Summoned without Tribute, allowing the user a more efficient way of bringing out monsters to inflict heavy damage.
Decrease Level Method 2
Another variation of this method utilizes stall cards such as "Gravity Bind" or "Level Limit - Area B" to halt monsters above Level 4. Since the user's WATER monsters (preferably monsters originally Level 4) are decreased in Level, then those monsters would be able to escape the stall cards. This allows "Mermaid Knight" or "Amphibious Bugroth MK-3" to either attack twice or attack directly. In the case of "Bugroth", "Limiter Removal" can prove useful. "Field Barrier" will help in protecting "A Legendary Ocean". Spell/Trap negation and/or removal will be handy as well, including "Dust Tornado", "Twin Twisters"/"Mystical Space Typhoon", or "Solemn Judgment".
"A Legendary Ocean" Decks to some extent can also utilize Synchro Monsters. "Deep Sea Diva" is a Tuner monster, and when Normal Summoned it can bring out a Level 3 Sea Serpent-Type monster from your Deck, such as "Atlantean Pikeman", "Mermaid Archer", or "Spined Gillman". This can allow a Level 4 or 5 Synchro Monster to be Summoned easily, although with "A Legendary Ocean" it will instead allow the Summoning of lower-Level Synchro Monsters. Additionally, "Deep Sea Diva" has less than 1500 ATK, so it can be added back to your hand by "Salvage".
"A Legendary Ocean" Decks to some extent can also utilize Xyz Monsters. Due to "A Legendary Ocean" lowering the Levels of WATER monsters, it is easy to Summon Rank 3 or lower Xyz Monsters, particularly WATER Xyz Monsters. Also, using original Level 5 WATER monsters whose Levels are lowered by "A Legendary Ocean", such as "Giga Gagagigo" or "Terrorking Salmon", allows the player to Summon Rank 4 Xyz Monsters, such as "Bahamut Shark" which can use its effect to Summon strong Rank 3 or lower WATER Xyz Monsters.
One weakness of this Deck is that in order for monsters to reach their full potential, they must have A Legendary Ocean or Umi on the field. This is easily remedied with cards such as Warrior of Atlantis. Another great weakness of this deck is Tualatin, as it can stop you from Summoning any monsters. Adding cards such as Horn of Heaven and Bottomless Trap Hole can help you overcome these hazards.
These decks also function very slowly. So it is important to have cards against your opponent for decks that might be faster than you.