(This card's name is always treated as "Umi".)
All WATER monsters on the field gain 200 ATK/DEF. Reduce the Level of all WATER monsters in both players' hands and on the field by 1.
Rulings
Individual Card FAQs
You can have 3 cards total in your Deck between "A Legendary Ocean" and "Umi", because this card's name is treated as "Umi". So you cannot have 3 copies of "Umi" and 3 copies of "A Legendary Ocean" in your Deck.
The name of "A Legendary Ocean" is treated as "Umi" always and for all purposes. If "Imperial Order" is active, the card name is still treated as "Umi". If the opponent activates "Cursed Seal of the Forbidden Spell" targeting "Umi", you cannot activate "Umi" or "A Legendary Ocean" for the rest of the Duel.
When "A Legendary Ocean" is active, you may activate "Tornado Wall".
When "A Legendary Ocean" is active, you may Summon or Set Level 5 WATER Monsters from your hand, since they are treated as being Level 4 monsters. If you Set a Level 5 WATER monster you do not have to show your opponent.
When "A Legendary Ocean" is active, "The Legendary Fisherman" on the field is unaffected by Spell Cards, so it does not receive the ATK/DEF bonus or have its Level reduced by 1. It still is treated as Level 4 in your hand, so you may Summon it without Tribute.
When the effect of "Cyber Jar" is activated and "A Legendary Ocean" is on the field, you cannot Special Summon Level 5 WATER monsters picked-up by the effect of "Cyber Jar". This is because the effect of "A Legendary Ocean" does not include Monster Cards in the Deck.
If "A Legendary Ocean" and "Gravity Bind" are active, you may attack with Level 4 WATER monsters (being treated as Level 3). If "A Legendary Ocean" is destroyed after an attack is declared, a replay occurs because the Level 4 WATER monster can no longer attack.
If "A Legendary Ocean" and "Infinite Dismissal" are active, and you Summon a Level 4 WATER monster (being treated as Level 3), it will be destroyed during the End Phase.
"A Legendary Ocean" does not have the effect of "Umi" that increases/decreases the ATK and DEF of specific monster Types.
Netrep Rulings
While "A Legendary Ocean" is on the field, Ritual Summons using WATER monsters as Tributes must take into account the reduced Star Level.
Cards revealed by "Cyber Jar" or "Morphing Jar #2" are not considered in the hand. The one star reduction given by "A Legendary Ocean" does not apply to monsters revealed by "Cyber Jar" or "Morphing Jar #2's" effect.
If "Prohibition" has been activated and "Umi" has been declared, "A Legendary Ocean" may not be played.
"A Legendary Ocean" is not a valid choice for the effect of "Prohibition."
While "The Legendary Fisherman" is face-up on the field and active while "A Legendary Ocean" is face up on the field, "The Legendary Fisherman" will be a 5 Star Monster as it is unaffected by Spell Cards, including "A Legendary Ocean."
Spell|
Field